if someone is seeding that video cause they recently watched it and it remains on their local cache, you could get the bits from there vs hitting youtube servers
The client can pick the optimal encoding and quality which makes that quite a bit more difficult. A user on a modern device that can hardware decode HDR av1 will have a file cached completely useless to a browser that can only do SDR h264.
I think there is a sort of network effect needed to make it become reasonable.
Netflix reported in 2025 something like 30% of traffic was av1. I think smartphone and PC support are fairly robust but under the impression streaming devices are still quite terrible unless it's a Chromecast.
AVC/h.264 is the only thing I know of that's pretty much universally supported at this point. I found a Facebook/Meta report citing some consultant provided data that VP9 was a close second. Limiting to devices from the last few years, HEVC might work.