It is ridiculous that GitHub has not even responded to your support ticket. To anyone reading from GitHub, this is making me rethink my choice of GitHub as a platform, and I'm sure the same is true for other people reading this post. Your reputation is very much at stake. To anyone reading this from Gitlab, how easy is it to migrate CI/CD off of GitHub Actions to Gitlab?
However this is just vendor management. You have to balance risk and productivity, and it's even more important now with how many different services a typical business uses.
Most people are better off using third-party services with proper support contracts rather than trying to plan for an edge case while ignoring all the other new challenges it brings to do it all yourself.