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They don't specifically say you can install your own OS but they have similar wording on the Steam Frame page as their other systems:

> Just like any SteamOS device, install your own apps, open a browser, do what you want: It's your PC.

But as gp correctly points out, with ARM there tend to be additional technical challenges for that compared to your normal PC even if Valve does nothing to intentionally lock it down.

Android phones with unlocked bootloaders are awkward, but doable. It would be somewhat un-Valve-like if they lock the bootloader.

Steam Machine and Steam Deck are open so I cannot see why this wouldn’t be.

Those are x86. But yeah it does sound like it will be open.