There are other submitted Merge Requests to that gnome-session GitLab repository that are 3 years old and are still open. This is only a proposal, and doesn't actually mean it's happening.
If GNOME developers want to focus on expanding Wayland support instead of maintaining X11 support, surely that's their choice - they're mostly volunteers anyway, shouldn't they get to decide what they want to work on?
If other developers still want X11 support, they can branch these session targets and X11 support code off into a separate package and handle maintaining it.
