given that lightdm, a much less featured login manager supports guest logins, kdm should at least support guest logins. There are no options to enable guest logins from the system settings>login manager Reproducible: Always
KDM is unmaintained and not used in KDE Plasma 5. SDDM is the login manager used in KDE Plasma 5. If you still have this same issue with SDDM, please file an issue on the SDDM bugtracker (after doing a search for existing issues first!): https://github.com/sddm/sddm/issues/