Well. It doesn't say in any docs or specs, but for what it's worth, Valve's hardware has always been open like that. You're free to install windows on your steam deck, for example.
They do hint that you can install a different OS on it:
> Just like any SteamOS device, install your own apps, open a browser, do what you want: It's your PC.
Every other SteamOS device does allow you to install whatever OS on the device, so seems Frame will be the same, judging by that landing page blurb.