YouTube bitrot is real in the sense that over the years, they've re-encoded everything multiple times, resulting in older content looking absolutely terrible. This includes dropping higher resolution options. They don't even seem to keep the original source files for subsequent re-encodes.

If you watch anything from a decade or so ago, you'd be forgiven for thinking that video content just didn't look very good back then, but no, it's largely down to YouTube compressing them to death since the original upload.