I have Plasma 5.19.5 on Solus 4.1 and I don't see any option to switch users. Not on the login screen, lock screen nor any of the (alternative) application menus.
I think it should be when registering so it is with me go to the system setting user account details and enter a new user then restart the system or does your system start without a password? Of course you don't see anything
It's a bug, probably in systemd.
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