Since no one has commented on this yet, I’m going to guess that it hasn’t been discussed very much yet and y’all want to hear my thoughts on it 
Blender’s bundled add-on development is a bit of a mess currently IMO, add-ons generally don’t have a central landing page where you can see what’s happening with them, issues and diffs are all lumped in together, if the development isn’t being conducted on some external repository somewhere, and developers with commit access can change bundled add-on code without the author’s knowledge or consent (this isn’t to say that contributions aren’t welcome, but it can cause problems like ⚓ T95962 Sapling Addon now broken because of "SPDX-License-Identifier" header).
Since it seems that Blender will be adopting the fork/pull request paradigm, my current thinking is that I would like to see each bundled add-on get it’s own repo to manage development (bugs, tasks, landing page, PRs, etc.), have scripts/buildbot incorporate the master branch from each of these into the daily builds/releases, and a central listing of bundled add-ons so that the repos are easily findable. Or perhaps bundled add-on development should move entirely to external repos on GitHub, GitLab, Codeberg, etc., but then there is the challenge of synchronizing them with the rest of Blender.
Now I know this doesn’t directly help with picking the new development platform, but I think this is a good time to discuss it and to keep add-on development in mind when looking at the different platforms in case one platform is better suited for something like this than another.
Also, how bundled add-on development proceeds will be influenced by the new policies that have been made recently, and that are hopefully being discussed and clarified here Add-ons Policy Clarification/Discussion so feel free to chime in there if anyone has any thoughts on that as well.