The move would be done by replacing each repository’s content with a readme.md pointing to the new repository location. This way it would still be reachable via github’s built-in search. Additionally, I don’t see why we couldn’t keep the existing issue tracker AND have issue trackers on multiple platforms (surely someone has already written a a bot to keep them in sync). We would also encourage people to use the new location’s issue tracker, but not mandate doing so. Additionally, having the repositories on multiple hosts would improve our search engine ranking.
Projects with good backup regimes in places should not fear some services going offline. Given how much activity there is on our online areas, the loss of a few hours of work would be at most a few dozen sentences. As far as I’m aware, we already have repository mirrors on gitlab. However these currently do not include the issue tracker and some other metadata.
There is also no reason preventing us from utilizing multiple new lower-reliability hosts (notabug,framagit,…) simultaneously, but we have to decide which of those to make our new default and setup automated sync to the rest of them.
As far as other services not being famous goes, the more we use them the more famous they become and the less we use github the less famous it becomes. Every repository counts, especially if we are loud about migrating.