How to perform a "Select all" via keyboard, in recent versions of Gnome Terminal?
It seems like that option has been voluntarily removed.
The closest I can get to get something similar is enabling "Enable mnemonics (such as Alt+F to open the File menu)" in Edit > Preferences:
And use ALT+E,↓,↓,ENTER to select all.
Enable e.g. Shift + Ctrl + A as the shortcut key for Select All in Preferences → Shortcuts.