SDDM will keep working, and KDE maintains a LightDM greeter. You don't even need to use a login manager, most people in the BSD space launch it from the tty

You mean people still log into a virtual terminal and call startx? (blushing emoji)

I mean, if that's the way they like it, that's fine. It feels a bit quaint, though, at least to me. If a graphical desktop is what I want anyway, why not make it automatic? I'm not judging, just trying to understand.