Realistically, you’re talking about what comes as default on the most-used distro.
I can’t remember the exact percentage, but the overwhelming majority of users don’t change any default setting they don’t have to.
Given that every major distro has multiple variants with different GUI/Windowing systems, that "most-used" version could easily be using a GUI that is only installed on a small fraction of the overall Linux userbase. The "one size fits most" label applies less to Linux than any other OS because of how much flexibility there is in the OS at the core levels.