Just realized the weight of moving the primary location of our source code given the number of links to it out there
And the whole forks ecosystem
And the way commit messages refer to GitHub issue numbers
@Gargron FWIW, you can move the repo and turn GitHub into a mirror.
GNOME did it before moving to GitLab, and it did improve contributions. (The recent move to GitLab majorly improved collaboration even more — but that's because they had relatively antiquated systems that still needed to be used beore.)
But, basically, you can move and keep the project on GitHub as a mirror. If you do so, I'd suggest having prominent notice in the README and links to the main repo in the README & project URL.
@Gargron GitLab has push mirroring for enterprise edition — does your Ultimate License also support it? https://docs.gitlab.com/ee/workflow/repository_mirroring.html
(Otherwise, a mirror can be just another git remote that is pushed to, and that can even be automated.)