I use a 1080p monitor and what I’ve noticed is that once creators start uploading 4k content the 1080p version that I watch on fullscreen has more artifacting than when they only uploaded in 1080p.
Did you notice that as well?
Watching in 1440p on a 1080p monitor results in a much better image, to the detriment of theoretically less sharper image and a lot higher CPU usage.
I am running mostly Firefox or Librewolf on Linux these days, but I do not remember having to enable it. Not all of my systems support accelerating AV1 in their hardware, but they do play at 1080p (but with framedrops once above 30fps on the unaccelerated computer). But yeah, I do hope YT keeps VP9 around because of the acceleration.