On FreeBSD, you either run the Windows build through WINE or the Linux build through the Linux emulator.
People have claimed that running the SteamOS UI on a non-Steam Deck is reported as Steam Deck, so I’m guessing it’s based on the client version, not the OS version. So my money is on “Windows” if you’re running through WINE or “Linux” if you’re using the Linux compatibility layer. The hardware survey probably says “Other.”
That said, I do have like 5% on macOS, which is basically BSD. So I guess I’m a meme. :)
But you got me curious, maybe I’ll install FreeBSD and see if I can get Proton to work properly. I love FreeBSD, and if I can get most of my games working, I may consider switching back. It was my first *nix and I tried it as a desktop for a couple months, but bailed because of sleep not working on my laptop and Steam not working well on my desktop. Maybe it’s worth another look.