This update is for the SteamOS Beta and Preview channels, and includes new features that are still being tested. You can opt into this in Settings > System > System Update Channel.
Note: This beta contains changes that were simultaneously released to SteamOS stable channel as 3.8.15, noted below
Known Issues - Beta
There is a known issue with degraded performance in some cases when composition is required (such as when having FSR enabled).
This issue is fixed in the latest SteamOS stable release (3.8.15). Affected users may wish to switch to the stable update channel while we work on bringing this fix to the beta channel.
General
Fixed delayed start time for some games under Proton when no capture device is present
Security and stability improvements
Fixed in 3.8.15
Fixed an issue with HDR in Assassin's Creed Black Flag Resynced