Interesting! Which theme are you using?
€: Just tried Fedora in a VM and also don’t have the switch user option. Switched the theme to the KDE Breeze Dark theme that I use on my normal machine → still no switch user option.
€: Found it:
added
[KDE Action Restrictions]
action/start_new_session=true
action/switch_user=true
to ~/.config/kdeglobals
like the last comment suggested and it works. ( to clarify the button appears I have not checked if it works without issues - proceed with caution!)