Bcachefs

As far as I know bcachefs doesn't implement the fscrypt API and only supports whole filesystem encryption, analogous to LUKS, so it doesn't provide the features Valve needed.

Also as far as I know, nobody's working on fscrypt support for Btrfs anymore, after years of back-and-forth with the fscrypt maintainer the last news of the project was an email by Josef Bacik saying that Meta didn't need the feature anymore so it had been de-prioritized but he'd try to fix up what had been done and submit it... but that was in April 2024 and he left Meta and stepped back from kernel development since.