But this commit was to the peps repo, which has far fewer
commits, one branch, no tags, and only 10 PRs. So while it's still
an issue, it's not as big a deal as if we were talking about the
cpython repo.
I don't know how many forks have been made since the commit in
question.
I don't know about new forks that are just sitting there, but GitHub has a handy network graph that shows where every fork has diverged from the main repo:
https://github.com/python/peps/network
If you look to the right of the line down to KeanaBerlin on June 26, it seems that six people have pushed commits to a fork that diverged after that commit (plus one that committed something to an old fork). That is based on the time of the commit to the fork, not when it was merged into the main repo, so it could be fewer, but in any case no more than six people would need to force-push to their fork(s) if a rewrite was done now. Also, some of those forks may have already been the subject of PRs that have been merged, and therefore may be able to be ignored.