avg@ reports to the mailing list (https://mail.kde.org/pipermail/kde-freebsd/2019-July/031951.html):
> Does user switching work for anybody?
> Is there any additional configuration needed to get it working?
> It does not work for me at all. When I click "Start New Session" I am taken
back to the unlock screen for a current user.
> This is with the latest plasma 5.16.2.
I haven't tested this with 5.16.5, but I guess it's still broken.
Yes it's still broken.
I should clarify that the original report was for sddm.
Later, I tried lightdm and with it "Start New Session" does indeed start a new X/lightdm session.