There are lots of unix boxes that serve many users. The mail server that
sends this does, without requiring each email user to have a unix account.
The only devuan server I have in production so far does, but the users are
in a postgres database, not in /etc/password.

There are also lots of unix boxes that have many UIDs. Don't android phones
assign a uid to each app you install?

The concept that's dying or dead is a login screen that asks "which of my
many longtime users are you?" and then serve a UI. That used to be common,
but nowadays corporate IT tends to give people laptops and each person has
files on that laptop, so the laptop is more a singleuser device than a
shared-among-whole-company thing. Even if a laptop is accessible to others
in principle because of Active Directory or similar, the local files hollow
out its multiseat capability.

Kiosks remain, but many of those don't usually have longtime users, you get
a freshly set up "guest" account when you start. I've also seen one that
VNC'd me to my own virtualbox guest instead of using unix/x's multiseat.

And of course sshd remains heavily used among many sysadmins, at least, but
that too doesn't have a lot to do with login screens or multiseat.

Arnt