X Server implementation vs Linux distributions
Linux has a lot of distribution – here is the popularity ranking. Do all Linux distributions share the same X Server implementation ?
Of course nobody can know all X server implementations nor all Linux distros. There will be distros for specific use cases and they will ship legacy software, because of some specific reason.
But to the best of my knowledge, there is only one fully-featured standalone open source X Server implementation that targets Linux and still is being supported, the X.org Server, and so every Foss Linux distro that tries to support X natively will have to ship exactly that.
Not that not every Linux distro has to support X natively. With Wayland, there is at least a partially viable alternative to X (and with XWayland you could even run X applications; but XWayland is also part of the X.org Server, so that’s also the same server). It is generally hoped that Wayland ecosystem reaches maturity, and that Wayland replaces the X protocol and architecture one day, getting rid of 40 years of questionable design decisions.
Tiny Core Linux, a Linux distribution that focus on being as lightweight as possible, uses its own fork of Xvesa, an old minimal alternative X implementation, called TinyX. It might not support all of X.org features, but it worked well when I used Tiny Core. You have to sacrifice some things to make a modern Linux distribution that fits on 11 MB of disk and that only requires 50 MB of RAM.
So, yes, it’s official, not all Linux distro ships with the X.org version of X11.
I’m also going to add that OpenBSD use its own fork of X.org called Xenocara. I don’t believe they change much other than what they need to make it compile, so not really a different version than the X.org everyone else uses.