Как получить данные из последней строки определенного столбца?

Попробуйте так:

import os                                                     
import csv                                                      
from PyQt5 import QtCore, QtGui, QtWidgets


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(407, 353)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout_3 = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout_3.setObjectName("gridLayout_3")
        self.tabWidget = QtWidgets.QTabWidget(self.centralwidget)
        self.tabWidget.setObjectName("tabWidget")
        self.tab = QtWidgets.QWidget()
        self.tab.setObjectName("tab")
        self.gridLayout = QtWidgets.QGridLayout(self.tab)
        self.gridLayout.setObjectName("gridLayout")
        self.tableWidget = QtWidgets.QTableWidget(self.tab)
        self.tableWidget.setObjectName("tableWidget")
        self.tableWidget.setColumnCount(8)
        self.tableWidget.setRowCount(0)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(0, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(1, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(2, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(3, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(4, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(5, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(6, item)
        item = QtWidgets.QTableWidgetItem()
        self.tableWidget.setHorizontalHeaderItem(7, item)
        self.tableWidget.horizontalHeader().setDefaultSectionSize(120)
        self.gridLayout.addWidget(self.tableWidget, 0, 0, 1, 1)
        self.groupBox = QtWidgets.QGroupBox(self.tab)
        self.groupBox.setObjectName("groupBox")
        self.gridLayout_4 = QtWidgets.QGridLayout(self.groupBox)
        self.gridLayout_4.setObjectName("gridLayout_4")
        self.pushButton = QtWidgets.QPushButton(self.groupBox)
        self.pushButton.setObjectName("pushButton")
        self.gridLayout_4.addWidget(self.pushButton, 0, 0, 1, 2)        # + 2
        self.pushButton_2 = QtWidgets.QPushButton(self.groupBox)
        self.pushButton_2.setObjectName("pushButton_2")
        self.gridLayout_4.addWidget(self.pushButton_2, 1, 0, 1, 2)      # + 2
        self.gridLayout.addWidget(self.groupBox, 1, 0, 1, 1)
        self.tabWidget.addTab(self.tab, "")
        self.tab_2 = QtWidgets.QWidget()
        self.tab_2.setObjectName("tab_2")
        self.gridLayout_5 = QtWidgets.QGridLayout(self.tab_2)
        self.gridLayout_5.setObjectName("gridLayout_5")
        self.groupBox_2 = QtWidgets.QGroupBox(self.tab_2)
        self.groupBox_2.setObjectName("groupBox_2")
        self.gridLayout_5.addWidget(self.groupBox_2, 1, 0, 1, 1)
        self.groupBox_3 = QtWidgets.QGroupBox(self.tab_2)
        self.groupBox_3.setObjectName("groupBox_3")
        self.gridLayout_5.addWidget(self.groupBox_3, 2, 0, 1, 1)
        self.tabWidget.addTab(self.tab_2, "")
        self.gridLayout_3.addWidget(self.tabWidget, 0, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 407, 21))
        self.menubar.setObjectName("menubar")
        self.menu = QtWidgets.QMenu(self.menubar)
        self.menu.setObjectName("menu")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.action = QtWidgets.QAction(MainWindow)
        self.action.setObjectName("action")
        self.action_2 = QtWidgets.QAction(MainWindow)
        self.action_2.setObjectName("action_2")
        self.menu.addAction(self.action)
        self.menu.addAction(self.action_2)
        self.menubar.addAction(self.menu.menuAction())

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(0)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        item = self.tableWidget.horizontalHeaderItem(0)
        item.setText(_translate("MainWindow", "№"))
        item = self.tableWidget.horizontalHeaderItem(1)
        item.setText(_translate("MainWindow", "Фамилия"))
        item = self.tableWidget.horizontalHeaderItem(2)
        item.setText(_translate("MainWindow", "Имя"))
        item = self.tableWidget.horizontalHeaderItem(3)
        item.setText(_translate("MainWindow", "Отечество"))
        item = self.tableWidget.horizontalHeaderItem(4)
        item.setText(_translate("MainWindow", "Адрес"))
        item = self.tableWidget.horizontalHeaderItem(5)
        item.setText(_translate("MainWindow", "Номер телефона"))
        item = self.tableWidget.horizontalHeaderItem(6)
        item.setText(_translate("MainWindow", "Дата регистрации"))
        item = self.tableWidget.horizontalHeaderItem(7)
        item.setText(_translate("MainWindow", "Активность"))
        self.groupBox.setTitle(_translate("MainWindow", "Настройки таблицы"))
        self.pushButton.setText(_translate("MainWindow", "Добавить строку"))
        self.pushButton_2.setText(_translate("MainWindow", "Удалить строку"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), _translate("MainWindow", "Таблица"))
        self.groupBox_2.setTitle(_translate("MainWindow", "Ввод запроса"))
        self.groupBox_3.setTitle(_translate("MainWindow", "Результат вопроса"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab_2), _translate("MainWindow", "Поиск данных"))
        self.menu.setTitle(_translate("MainWindow", "Файл"))
        self.action.setText(_translate("MainWindow", "Открыть"))
        self.action_2.setText(_translate("MainWindow", "Сохранить как..."))
        

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        self.pushButton.clicked.connect(self.buttonAdd)
        self.pushButton_2.clicked.connect(self.buttonDelete)
        self.action.triggered.connect(self.actionOpen)          
        self.action_2.triggered.connect(self.actionSave)        

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
        self.spinBox = QtWidgets.QSpinBox()
        self.spinBox.setPrefix(" Введите индекс колонки ->   ")           
        self.spinBox.setRange(0, self.tableWidget.columnCount())
        self.pushButton_last = QtWidgets.QPushButton(
            'Значение последней строчки определенного столбца')
        self.pushButton_last.clicked.connect(self.buttonLast)
        
        self.gridLayout_4.addWidget(self.spinBox, 2, 0)
        self.gridLayout_4.addWidget(self.pushButton_last, 2, 1)

    def buttonLast(self):
        row = self.tableWidget.rowCount()
        if not row:
            return
            
        column = self.spinBox.value()
        item = self.tableWidget.item(row-1, column)
        if not item:
            msg = QtWidgets.QMessageBox.information(
                self, 'Внимание', 
                f'строка {row}, колонка {column+1} - Пустая')        
        else:
            item = item.text()
            msg = QtWidgets.QMessageBox.information(
                self, 'Внимание', 
                f'строка {row}, колонка {column+1} = {item}')             
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        
    def buttonAdd(self):
        rowPosition = self.tableWidget.rowCount()
        self.tableWidget.insertRow(rowPosition)

    def buttonDelete(self):
        if self.tableWidget.rowCount() > 0:
            self.tableWidget.removeRow(self.tableWidget.rowCount() - 1)

    def actionSave(self):
        path = QtWidgets.QFileDialog.getSaveFileName(
            self, 
            'Save CSV', 
            os.getenv('HOME'), 
            'CSV(*.csv)')
        if path[0] != '':
            with open(path[0], 'w', newline='') as csv_file:
                writer = csv.writer(csv_file, dialect='excel')
                for row in range(self.tableWidget.rowCount()):
                    row_data = []
                    for column in range(self.tableWidget.columnCount()):
                        item = self.tableWidget.item(row, column)
                        if item is not None:
                            row_data.append(item.text())
                        else:
                            row_data.append('')
                    writer.writerow(row_data)

    def actionOpen(self):
        path, _ = QtWidgets.QFileDialog.getOpenFileName(
            self, 'Open CSV', '.', 'CSV(*.csv)') 
        if not path:
            return
            
        with open(path, newline='') as csv_file:
            self.tableWidget.setRowCount(0)
            self.tableWidget.setColumnCount(8)
            my_file = csv.reader(csv_file, delimiter=',', quotechar='|')
            for row_data in my_file:
                row = self.tableWidget.rowCount()
                self.tableWidget.insertRow(row)
                if len(row_data) > 8:
                    self.tableWidget.setColumnCount(len(row_data))
                            
                for column, stuff in enumerate(row_data):
                    item = QtWidgets.QTableWidgetItem(stuff)
                    self.tableWidget.setItem(row, column, item)


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.resize(1020, 353)
    w.show()
    sys.exit(app.exec_())