By the time that the vast majority of users move to Wayland, it will be quite obvious that any X11 usage has declined, purely due to the discussions on the comm

By the time that the vast majority of users move to Wayland, it will be quite obvious that any X11 usage has declined, purely due to the discussions on the community grapevine attesting to that fact. To decide whether a platform needs to be dropped, you don't need anything as fine-grained as telemetry. If you want to be conservative in your estimates (which, if you care about backwards compatibility, you should be), then it will become quite obvious that no-one is using a particular platform any more.