The community and their own time dealing with said community would both be fragmented. I have a feeling what they most want to do in maintaining the project is to spend time maintaining the project, not the infrastructure for distributing and supporting it.

They can at a bare minimum point to say an IRC #room on Libera.Chat and say it's unofficial, so folks can know where to congregate. In the case of Mastodon or other Fediverse, make that the official and have that forward to Twitter if you cared. GitLab (which isn't the only alternative Git forge) itself lets you add to merge requests via patches and emails--along with supporting automatic mirroring. You can make one just a read-only archive. You can also just support two platforms--or drop the problematic proprietary one.