Ye, some of these i started thinking of after i made my comment, which is my bad. Its true a project that uses the full github stack is harder to move, its its still relatively easy. The only problem you’d have is redirecting traffic to a new host, but this problem exists in all platforms and not only in project management sites like github.
As for your bulletpoints, i have a few remarks. Mainly that github pages are silly and they should not be used as a website. And even if you are, there are tools out there to convert the markdown from github pages into html/css/js so moving that is easy. Same with wiki pages, they are just markdown. 90% of markdown is compatible with other sites, and the parts that arent probably have a site specific syntax that youll need to look up, nothing bad. Review comments i will disregard, as those are part of the PR process. Once a project is moved you could hide the PR tab on github or close all new ones redirecting them to the new host. All older ones can be handled and phased out. Once a pr is merged, get the commit from github and push it to the new host. Thanks git!
Random contributers have nothing to say imo. If a project’s team feels the need to move away from github a true contributer/side team member/helper will move with them, and if they dont then so be it. Once a project team feels the need to move, something bad must be going on and moving will always be the better move for their mental health than to keep working with bad stuff.
As mentioned before ci/release pipelines are all yaml. Their odeas stay the same, only action names & their patameters change. Nothing complex there to move there…
So im short : only moving your traffic is a real problem, but is a problem on all websites and all communities, not just github or a project on github