What it looks like if it's available (AV1 settings section): https://i.imgur.com/rp6Cvkd.png

The wording is gold dark pattern.

"auto"

"prefer AV1"

"always prefer AV1"

so... which one turns av1 off?

None, but "prefer AV1 for SD" will prevent the stuttering due to the changes mentioned above.

If you don't ever want AV1 then it probably makes more sense to not have the browser advertise it as an option to sites. One can configure Firefox as such, I'm not sure about Chrome.

There is an "AV1 settings" below "Subtitles and Closed Captions" for me. If you don't see it perhaps it's not available for you yet.