The quit menu is a special "menu" that allows to quit the game and exit ZDoom. Its MENUDEF name is QuitMenu and it can be summoned with the menu_quit console command (bound to the "F10" key by default). It displays a random, game-dependent quit message and asks for confirmation.
In vanilla Strife, the equivalent function played a random sound of Blackbird mocking you for quitting after confirmation and before exit. Because this behavior was found to be annoying (quitting should be instantaneous and shouldn't force the player to wait), ZDoom does not play these sounds; instead transcriptions of these jabs are used as the confirmation messages for this game.