I don't think we're seeing a Github decline. Yes they've had some internal strife not all that connected to business, but Google just moved a lot of its OSS there, Microsoft just moved a bunch of stuff there. Git repos take a single command line statement to move, but Github is only slowing if by the force of not having much left to hoover up.

Gitlab et al are growing, and now AWS has its own integrated solution, but I don't see Github going anywhere for a very, very long time. (Barring catastrophic happenings, of course.)