I did not look into the details. There is a warning at the top of the page, and stuff that has worked before, like previewing markdown in comments, is broken.
Best guess: they removed a polyfill somewhere.
Best guess: they removed a polyfill somewhere.
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.