This is a little exciting! It seems Valve are pushing ahead with some major updates to SteamOS with SteamOS 3.0.
Looks like it will be codenamed Clockwerk, continuing their tradition of naming their versions after Dota 2 characters like we had with the previous Alchemist and Brewmaster releases. You can see their repo list here, which shows both a Clockwerk and Clockwerk Beta listing.
Valve actually hinted that this was coming with the last update to SteamOS, where they noted that they had updated their build infrastructure.
It's possible this will be based on Debian 9, which was released back in 2017 since Brewmaster is still based on Debian 8 which was released way back in 2015. Update: Turns out it is indeed based on Debian 9, as pointed out in our comments.
I wouldn't expect major changes in how SteamOS actually operates though, as this will likely involve Kernel and GPU driver updates to help SteamOS keep up with modern hardware. It does show they still believe in it though, slow and steady wins the race? Will be fun to see exactly what it includes.
For the current stable version of SteamOS, they just pushed the previous beta out to everyone too with SteamOS update 2.154.
Hat tip to dubigrasu.