I think sometimes people forget that one of the main features of Git is that it's decentralized. You don't need Github; just push your repo to a different remote.
Everyone that clones the repo (usually) has a full copy of it, including all history, and theoretically you can clone the repo directly from their copy. Of course, that's often not practical, which is how we ended up with these centralized services.
The main issue with losing a Github repo is the auxiliary non-Git-powered features of Github, like issue tracking.
50TB on a Hetzner storage box would be $116/month, with unlimited traffic. It'd have to be split across three storage boxes though, since 20TB is the max per box. 10TB is $24/month and 20TB is $46/month.
They're only available in Germany and Finland, but data transfer from elsewhere in the world would still be faster than AWS Glacier.
Another option with Herzner is a dedicated server. Unfortunately the max storage they let you add is 2 x 22TB SATA HDDs, which would only let you store 22TB of stuff (assuming RAID1), for over double the cost of a 20TB storage box.