`set-charset-registry' function itself exists in all 21.* branches.
However, its description was added to `lispref/mule.texi' in revision
1.4.2.1, but the change was never ported into release-21-{1,5}
branches. Moreover, `release-21-5' tag points to a revision rather
than a branch, and `r21-5-*' tags point to revisions from completely
different branch.