Quick answer: Valve's October 22 Steam Client Beta adds an HDR setting for Remote Play, HDR capture from desktop Linux and macOS, HDR on Android 15 phones with the beta Steam Link app, and re-enables HDR on Steam Deck OLED.
Valve pushed a new Steam Client update to the Beta channel on October 22, and this one is all about Remote Play. Valve calls it a big revamp of HDR support when streaming games from one device to another.
There's a new advanced client setting for HDR with three options. Automatic turns HDR on when HDR content is enabled, On keeps it on all the time, which Valve says allows increased brightness for SDR content, and Off turns it off.
Where you can stream from has grown too. Remote Play already handled HDR in games and on the Windows desktop, and now HDR capture is also supported when streaming from desktop Linux using PipeWire, and when streaming from macOS.
On the receiving side, HDR is enabled again on the Steam Deck OLED. It's also enabled on Android phones running Android 15 or newer, as long as you're using the beta version of the Steam Link app from Google Play.
For now these changes are only in the Steam Client on Valve's Preview/Beta channel.