KWin has supported input-method-unstable-v1 for quite a while

[tmke8]

Initial MR: https://invent.kde.org/plasma/kwin/-/merge_requests/106

Interface file: https://invent.kde.org/plasma/kwin/-/blob/master/src/wayland/inputmethod_v1.h

[SUPERCILEX]

If you're on kwin, you need to update this file by installing wlprobe and running wlprobe --unique.

[chirsz-ever]

Kwin only exposes relevant interfaces (zwp_input_panel_v1 and zwp_input_method_v1) to programs that are set as input methods.

https://invent.kde.org/plasma/kwin/-/blob/b97c6af859e49bebd8ec93889315c59fa31e56a3/src/wayland_server.cpp#L156