I'm not so relaxed about how easy it will be to move to a new issue tracker.

When you're moving from a self-hosted system to a proprietary system, you can do the work yourself to make sure there are redirects from the old URLs to the new ones.

That option isn't likely to be available when you're moving the other way.

I observe that github is happy for you to "bring your own domain" for github pages, but AIUI issue and PR URLs are always under github.com .