YouTube has flipped the switch and now serves up videos in the new AV1 codec on compatible Android TV devices. The royalty-free video format is gaining traction.
YouTube and AV1
Google, Netflix and others have committed to adopting the AV1 video codec developed by the Alliance for Open Media, founded by the same companies along with other industry heavyweights.
Netflix began streaming in AV1 in February 2020 and now YouTube has flipped the switch for compatible Android TV devices through the latest software update. YouTube has been experimented with AV1 in PC browsers for some time.

At this time, compatible Android TV devices include those the running on Broadcom's BCM72190/72180 and Realtek’s RTD1311/RTD1319 chipsets, according to XDA Developers who first spotted the change.
We were not able to pull down AV1 video streams on our 2019 Nvidia Shield, which is hardly surprising as it does not have hardware support for AV1. At this time you are unlikely to see any major changes.
However, in the long term AV1 support could bring 4K and HDR streaming to many more devices and streaming services following the slow roll-out of the royalty-based HEVC format as well as VP9, the two codecs currently being used for 4K HDR streaming. The royalty-free AV1 format is backed by Apple, Facebook, Netflix, LG, Microsoft, Netflix, Samsung and many others. LG and Samsung are also introducing the first TVs with AV1 support this year.
- Source: YouTube via XDA Developers