Is there any equivalent to GNOME’s Kiosk mode in KWin or Plasma? I assume this is possible but have not found any documentation for this, yet
I am aware of the KDE Kiosk mode but that seems to be about restricting functionalities and web access in KDE Frameworks/Apps to prevent breakout. What I am looking for is a single application mode similar to GNOME-Kiosk, cage, or Weston’s kiosk-shell.
The KDE kiosk configuration is documented here: Introduction to Kiosk | Developer
Yes I read that page but it seems more like access restrictions for childs etc where there should still be a full desktop environment. There does not seem to be any configuration to ensure one app is always started and always in fullscreen. I do not need a full plasma instance - just KWin starting a single application in fullscreen
I don’t think we have anything like that.
There are ways to get close to that result I think, like creating a session file for SDDM in /usr/share/wayland-sessions that just runs a single window with kwin_wayland. This way you can just select the session in the login screen.
We do have Plasma Nano but I can’t find much documentation on it. But like Herz pointed out, you can’t get much simpler than running KWin and having it launch your application.
That sound more like what I want 
I do not need SDDM but I guess it is also possible to start kwin without SDDM? (Otherwise I could use autologin but that would only slow the boot process)
I also know that there is kwin_wayland and kwin_wayland_wrapper. Which one would be the correct to choose?
That also sound interesting but yeah it is lacking documentation (and build instructions so I cannot easily try it out) 
PS: Is there a way to disable the forced emojis? I prefer the old school ASCII ones. Searching only turns up info about disabling the autocompletion but not the automatic conversion
I am not sure if this is a distribution issue but plasma-bigscreen also has no UI, i.e. it is solid gray which kinda seems to go in the correct direction (but raw kwin is likely better for me). Also there does not seem to be a release of it based on plasma 6 yet?
You want kwin_wayland. The wrapper is used for restarting the session in case of a crash.