https://9to5google.com/2018/09/14/youtube-av1-video-beta-chrome/

Users on Chrome 70 and Firefox Nightly builds after September 13th can test it by making sure media.av1.enabled and media.mediasource.experimental.enabled prefs are set.

chrome://flags/#enable-av1-decoder

Once running a supported browser, users can head to YouTube’s TestTube experiments list and select “‘Prefer AV1 for SD.”

2ヶ月前からAV1のテストが開始されてたんだね。