KDE Live can be installed without creating a regular user, only root. All you need to do is to not create a regular user in anaconda (only root), and then not create a regular user in the first boot utility (just click Finish). In SSDM, it is then possible to log in using the root account (not a good idea in general, to be sure).
Is this intentional, or a bug? Should KDE require creating a regular user, similarly to Workstation?
I believe this is part of the fundamental difference between KDE and GNOME.
GNOME locks you into things and you have no control over alot of things.
KDE allows you much for flexibility and lets you create your machine your way.
The Fedora Workstation Live CD install has modified the installer so you have a very limited set of things you can do. (3 things)
The KDE Live CD allows you to configure your machine more how you want (6 things)/
I love the "all you need to do" in your wording.
In reality, you have to create a root user during install (which is not default), and not create a user. So you've made a choice there.
You then have to not create a user on startup, it is literally right there. It is easier to click the create user than the Finish button. Again, another choice.
When you get to the SDDM login screen, root is not listed. No user is listed. So you have to know to put in 'root' and the password.
At this point, you have shown that you know who root is, and have deliberately setup your machine to run only as root.
So in reality the question you are asking is this.
"Is it ok for root to login and run graphically?"
I'm not saying this question is not important, it is important.
But, this is a very old question, and one that GNOME and KDE diverged on years ago.
Currently, KDE allows you to login graphically as root.
That is intentional.
p.s. I'm not saying it won't change, that's what I said "currently"