I mean why not? Was there an original SRD 5 prior to 5.1? I don’t have the OGL in front of me, but I don’t remember a clause in it regarding authorisation or revocation of the SRD. In fact didn’t SRD’s come out after the OGL?
The SRDs are released with a copy of the OGL attached. In that combination, the
OGL is the bit that grants rights to reuse it. The SRD itself does not grant permission to use it.
The concept of "revoking the SRD" has no legal meaning, it'd be like me saying "I revoke oxygen", there's nothing
to revoke, it grants no legal rights or obligations to anyone, it just exists as a copyrighted piece of work. They cannot delete copies of it from existence (other than the ones they themselves hold), because it's out there, in the wild, in multiple copies, all released under the OGL. They can remove it from their website, and I can just put it up on
my website for everyone to reuse, because the OGL gives me the right to do so.
The only part of the SRD that has any legal basis is the OGL at the back of it, which is the thing that grants us the right to use the Open Game Content within.
To put it another way, if you take the SRD to a Judge during a trial over its usage, the only bit they cares about reading (other than to see if text from it was used) is the OGL pages at the back, as that's the only bit that has anything to dispute.
Yes, I think there may have been a 5.0, there was also a 3.5 and a 3.0, none of which are invalidated by later releases. If you want to go dig up the 3.0 SRD, the OGL at this moment in time still allows you to use that rather than 5.1