This is too great not to share. Wayland devs hate this trick! I'll copy what I did from the bug report.

As a workaround you can use https://github.com/Supreeeme/extest to make Onboard work. Compile it as a 64 bit library and launch onboard with

env LD_PRELOAD=/usr/lib64/libextest.so onboard

If you want to use it with KDE you can add

X-KDE-Wayland-VirtualKeyboard=true

to its desktop-file.

I used kwin rules to get rid of window decorations and have it always on top without stealing focus. If someone knows how to make all other windows smaller when it's active that would be great.

Only problem remaining is that sometimes the keys get stuck on touch input. At least on my Steam Deck on OpenSUSE.

Edit: Just noticed that it doesn't work on KDE's lock screen. Hopefully I can find a workaround for that as well.

Edit 2: Was easier than I thought. Just select Maliit as a virtual keyboard and start Onboard manually. If you tap with your finger in a text field Maliit will come up. When you click in a text field Onboard will open. But Maliit also works on the lock screen.