I'm maybe missing something, but: the article states that Android previously provided an AV1 software decoder. New versions of Android ship a new software decoder called "libdav1d", which is currently opt-in - apps that don't opt in will get the old decoder. Youtube has opted in to using the new decoder.

But… doesn't this just mean that devices that were previously decoding AV1 Youtube streams in software will now decode them using a more efficient decoder? It doesn't say anything about Youtube changing the default stream format. Am I missing something?