Title is misleading. KDEs new login manager is systemd only, but you don't need to use it and can use any other login manager or no login manager just fine. They're replacing SDDM since they can have the login manager be more integrated if they're the sole user. They'll still be maintaining SDDM and LightDMs KDE greeter if those are more your cup of tea, and you can always just use startx (or whatever the wayland equivalent is)

Title can be misleading, as KDE still works on non-systemd systems that use other login managers, as discussed here: https://news.ycombinator.com/item?id=46870751

> niche Linux distros that would be cut out including everything from Gentoo to Alpine Linux and Slackware.

Gentoo lets you choose the init system (including systemd) so it doesn’t belong in this list.