This misses the point in a bad way. You shouldn't have a single deploy funnel to begin with. You should be able to do deploys from multiple places.
Lots of things we should do, only a few we actually have time to fix. If I were choosing between "Migrating away from GitHub" and "Adding another way to deploy in case GitHub is down", I'd take advantage and do the first, because you'll end up having to replicate SCM, build infrastructure and so on anyway, why not do it properly?
This misses the business reality in a bad way. A typical outage with something like Github is not a big deal for most businesses. Sure, some nerds get annoyed, but that's about it. The extra effort in maintaining multiple deploy approaches for a non-trivial system simply doesn't make business sense in most cases.
It's also something that's easy to do if you start from the beginning treating your CI/CD system as something that just runs scripts out of your repo.