[rascul]

All of them that I can remember. Note that this is from the '90s. At some point they all merged the X stuff into /usr. Maybe it was with the x.org switch, as XFree86 does indeed mention /usr/X11R6 in the docs.

https://xfree86.org/4.8.0/Install4.html#6

[mhd]

Ah, you meant as a directory. Sure, I remember that, I just thought you meant as a partition.