It's supposed to be royalty free, but there are patents for the techniques used in it which are used defensively. If someone has a patent to a technique used in AV1, then they could still demand royalties and some patent trolls have been trying. Wikipedia has a section on it.
Wasn’t part of AV1’s charm that anyone trying to sue an AV1 licensee would be nuked from orbit by the legal expertise and patent trove of the collective AV1 companies / AOM?
https://en.wikipedia.org/wiki/AV1#Patent_Claims