The FSF has no such requirement or even preference. In fact, we have tried multiple times to get this changed, but some GNU folks prefer CVS.
We do have painfully limited staff time, but if someone wants to implement other VCS for web pages, I personally would be extremely happy. We would not stand in the way and would do our best to facilitate (after the new server deployment is done). (GNA was not run by FSFE btw.)
