"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.