Tangentially related: GitHub has already dropped support for Mojave. The whole site is now broken in Safari 12 because it’s no longer the “current” version of the browser. Well, it is if you run Mojave, as you need to upgrade the entire system to get Safari 13. I can understand not developing new features for Safari 12, but speaking of regressions, why would you go break things that used to work a week ago?
Is that a message Github have put out, or have they accidently rolled out a new feature that has a bug in Safari 12? That seems more likely to me. Github's browser support is usually very good, and it'd be incredibly disappointing if they've broken something intentionally.