Users can choose h265 for live streams and they allow hevc uploads, but they then transcode it to worse codecs before broadcast.

I wonder how they would save on bandwidth by switching to hevc? I think its something like 40% more efficient on average.

I guess av1 is even better, but what percentage of hardware supports it?

Pretty much everything modern except Apple. Intel since 11th gen (2021), AMD since Zen4 (2022), Samsung phones since 2021, Google phones since 2021, Mediatek since 2020.

With modern lifecycles the way they are, that's probably ~60-80% of everything out there.

Also software decoding works just fine.