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.
For AV1 that could still be okay, lol. It would be kind of meh for e.g. H264 but YT does not even use that anymore AFAIK.
Youtube uses VP9 for all resolutions most of the time. 1080p and below offer AVC as fallback encoding