I clearly denoted a reason to keep git.sailfishos.org running for an additional while in the section “Example for this issue” in my original post above: To keep references to issues and merge requests working a little longer. For enabling Jolla employees and others to comprehend the context of issues migrated here (to FSO).
That sure implies to wait with activating the rewrite rules, or to explicitly exclude the URLs of issues and merge requests from being rewritten by the rewrite rules; as they were not migrated, that also makes sense in the long run (even after switching off git.sailfishos.org).
Hence the latter (“rewrite rules, which exclude issues & MRs”) would be the best solution, if that is sufficiently easy to implement (should be, IMO).