System Integrity Protection in macOS prevents processes writing to system locations.
There is currently a "compatibility bundle" at this location:
These appear to be system locations that processes can write to even with SIP enabled.
When I install Xquartz I see that in the post install script its builds a font cache and puts data in these locations (that part of that compatibility list):
If Apple were to remove these entries from the list in the future I think that would cause the post install script to fail, which would fail the entire install. This seems likely to happen.
Could this data be relocated elsewhere?
Not without breaking binary compatibility with shipping 3rd party apps, which is why we have them listed there.
If we ever remove the compatibility exceptions for some reason, X11 will stop finding fonts in those paths when using the older font system. The newer font system will continue to find them because fontconfig stores that data in an external cache.