Some critical issues have not yet been raised, which can all be encapsulated in the form of the GNU Ethical Repository Criteria. There are also some evaluations by these criteria, although the list of evaluated sites and software is far from complete.
Blender is a flagship free software program and should be a shining example of freedom. Relying on proprietary software, no matter how much the proprietors claim to be friendly to “open source” or want to give a discount for their proprietary software, is a complete betrayal of this goal of leading by example. No matter whether the entire platform has critical freedom issues (and privacy issues for that matter), eg. Github, or whether the software is only partially free, eg. GitLab, anything less than 100% freedom is a joke. Both of these sites currently score an F (unusable) in the GNU ERC, the worst possible grade.
Should the Blender project self-host or use an existing platform? Ideally the former, as this allows the Blender project to control its own infrastructure, software, and computers. The latter is okay, but not ideal, as it would mean the Blender project would lose a lot of the aforementioned control.
In any case, contributing without an account, anonymously over Tor, and without running JS, especially not proprietary JS, are an absolute bare minimum, which as I mentioned previously, is all encapsulated in the GNU ERC.
I vow to delete this account once this public discussion is concluded, and I resent having to make it in the first place in order to participate in this discussion. If I refused to make a temporary account, who knows whether anybody would have raised the issues I have, and as a result the final decision may have been disasterous.
Forges which allow communication by email are a solution, which leads me to lean towards self-hosting SourceHut as a preferred option, although it has a few issues which need fixing first, namely that it recommends non-libre OSs in its CI features and that it has not had an official “stable” release yet, although it’s already rock-solid.