kdesu under wayland (no matter if run from within plasma wayland session or gnome wayland) doesn't do anything.
Under X (tested in plasma and gnome) kdesu does what I want to use it for - run an application as a different user.
STEPS TO REPRODUCE
1. login to a wayland session as user1
2. run e.g. konsole as user2 with the command
kdesu -n -u user2 konsole
kdesu simply returns without opening a konsole window
A konsole window with user2 as process owner is spawned.
Like it works when run from within an X session.
Linux/KDE Plasma: 5.20.0
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1
Distribution: archlinux, latest packages.
Yes, I'm afraid Wayland's security model doesn't permit running apps as root, with elevated permissions, or as a different user like this. The app itself needs to gain whatever functionality you're using kdesu to approximate.