Want to deliver a handheld with the latest Windows (11) without their “AI button”?

Microsoft doesn’t (currently) require the AI button on laptops, much less on handhelds. Your point otherwise stands, though.

If Valve make a change that people don’t like… you can edit SteamOS in your own version and keep going with what Valvdin’t.

Effectively yes; technically no. If you’re calling the OS on your device “SteamOS” then you must have an agreement with Valve, and that agreement might restrict you in this regard. If you’re not restricted, then you can’t call it SteamOS - see the AyaNeo Next Lite for an example of this. It was announced as having SteamOS but then that was walked back; it’ll be running a fork, HoloISO, instead. You can see that hinted at here:

support for your devices has to be integrated into SteamOS ahead of time. This requires coordination between you, your device vendors and Valve well ahead of the release (or even specification) of your product.

Now, you might say that this is just semantics and that it won’t have any real impact. If you did, I’d disagree. This is relevant because this fragmentation will hinder brand recognition, particularly if multiple third party “forks” of SteamOS end up getting used, both in that those devices won’t be able to benefit nearly as much from the brand recognition of SteamOS and that if one of those devices takes off, SteamOS won’t benefit as much, either.