It is my understanding that XACE doesn't actually provide any security features itself. It just provides the "hooks" to implement security extensions. Like LSM feature in Linux kernel. You have to install a additional X11 extension to do something useful with it.
So the most common X11 security extension is going to be xcsecurity which enables the SECURITY extension. It allows a course permission model were applications can be designated as "Trusted" or "Untrusted". That is going to show up in many Linux distributions.
However all applications default to "trusted" because if they are untrusted they tend to cause lots of other annoying problems and crashes a lot of apps, apparently.
In practice the only place it shows up is if you are using "ssh -X". That uses the security extension by default. Which is why there is also a "ssh -Y" that disables it for applications that it breaks.
This sort of thing is why to fix X11 security you have to give up backwards compatibility and create a new X version.
Oh, wait, that is what the X developers did with Wayland.