It's now realistic to install Windows on a Steam Deck, provided you're wiling to live with certain limitations. Valve has released Windows drivers for the handheld's Bluetooth, graphics and WiFi, helping you use the system properly if the Linux-based SteamOS isn't to your liking. The developer has also shared instructions on how to install Windows on its gaming machine.
As you might gather, though, it's not a simple process. Audio drivers remain "in the works," so you'll have to rely on a Bluetooth or USB-C audio device. You'll have to install Windows 10 (Windows 11 support is coming through a BIOS update), and there's no dual-boot option at the moment — you'll have to replace SteamOS entirely.
Valve warned that it can't provide support for Windows users. You can revert to SteamOS using a USB recovery drive if necessary.
The limitations might make a good case for buying a Windows-native alternative like the Aya Neo or GPD Win 3. If you prefer the Steam Deck hardware and don't mind the lack of a safety net, though, this might be the moment you've been waiting for. Windows not only promises more performance thanks to native code (no Proton here), but access to Destiny 2 and other games that were previously off-limits.