Google has delayed the launch of Google TV user profiles so the December 2021 update is a minor one with security updates, improvements for HDR playback, and bug fixes.

Still on Android 10
There is a new October 2021 security patch but Chromecast with Google TV is still running Android 10 after the new system update. Google recently released Android TV 12 but has not commented on its plans to update Chromecast beyond Android 10.

The December 2021 update includes improvements for HDR playback, specifically proper HLG (Hybrid Log Gamma) support, which is used by services like BBC iPlayer (must be updated) as well as improvements for Dolby Vision.

The changelog also mentions DRM improvements in case you were getting black screens or video playback errors, "storage optimizations", and the usual "bug and performance" fixes.

In October, Google announced user profiles for Chromecast and other Google TV devices. The functionality was supposed to be made available this year but has been delayed and will instead be rolled out in "the coming months". 

 Also read: Review: Chromecast with Google TV

The new 158MB system update (QTS1.210311.036) is available now on Chromecast with Google TV.

Chromecast system update (QTS1.210311.036) changelog
- Security update: Android security patch level updated to October 2021
- Storage optimizations and management improvements
- Platform improvements to help some apps with: HDR HLG playing instead of converting to HDR10, Dolby Vision playback, and DRM Video decoding performance improvements.
- Additional bug fixes and performance improvements
 FlatpanelsHD