The shortcut Ctrl+Shift+Underline (_) is to remove the outline border from the selected cells. Please refer to shortcut Ctrl+Shift+Ampersand to add outline border.
Step 1: Open the file you want to remove the outline border;
Step 2: Select the data range with the outline border;
Step 3: Press the "Ctrl" and "Shift" keys from the keyboard, then press the underline sign from the keyboard to remove the border.