For users of older window managers on newer distros where X is likely to be removed "soon-ish" (vs LTS distros), the answer is to just use Wayback. It is a different X11 server implementation that uses Wayland under the hood, and is enough to run your WM. There are already people who use it to run AwesomeWM, IceWM, etc: https://gitlab.freedesktop.org/wayback/wayback

That said, you cannot run Wayland apps this way, it is purely for X11 only. If applications/rendering toolkits remove X11 support, you're out of luck. But Wayback may actually be a reason for some of them to keep maintaining X11 backends a bit longer, anyway.