Onboard is old news.

If you want to run Steam’s keyboard in your own Linux distribution on Wayland you need to have ibus running.

/usr/bin/ibus-daemon -d -r --panel=disable --emoji-extension=disable

Maybe you can also set some env variables to make it appear automatically. I’ll have to test how that clashes with KDE’s own virtual keyboard handling.

These are the variables I found:

# Set input method modules for Qt/GTK that will show the Steam keyboard
export QT_IM_MODULE=steam
export GTK_IM_MODULE=Steam

For more goodies look in /usr/bin/gamescope-session. It starts steam with everything needed to work on the Deck.