A recent video leak shows Microsoft has been working on ways to improve Windows functionality on handheld PC devices such as the Steam Deck. At least, it has been encouraging developers to come up with experimental ways to get Windows to play nice on handheld devices (via Windows Central).
There are a few reasons we've been sticking with SteamOS when it comes to the Steam Deck, not least due to the lack of full driver support for Microsoft OS. There are weird little bugs that crop up when using Windows on a handheld, too, not to mention the gaming performance on Windows is hardly comparable to playing through the SteamOS. Still, it looks like Microsoft is on the case, at least when it comes to the UI side of things.