Now with Steam Link client available on Linux, the only thing that may stop us is the default X11 streaming back-end.
Major DEs and window managers are now at least using wayland by default, if not ditching X11. The --pipewire flag should be enabled by default and to not leave legacy systems without it, make the --no-pipewire flag.

Pipewire can capture windows either on X11 or Wayland.