Updated SteamOS partner guidelines seem to point to an imminent full-scale launch of SteamOS on other gaming handhelds out of the box, nevermind simply as an ISO for gamers to install on their own machines. Previous work suggests the Asus ROG Ally may be among the first to receive "Powered by SteamOS" certification.
They’re less open about it but you can bet they’re still at it. Any OEM that sells anything but Windows will be penalised in licensing deals.