If it's anything like the Deck, then the version of SteamOS on it won't be locked down in any way whatsoever. You can install Windows or any other distro you want on the Deck with 0 issues (other than regular ones you'd experience anyways on any regular computer, nothing to do with Valve locking anything down).
The steam deck was not arm. Unlike the steam machine page, the steam frame page does not insinuate you can put a custom OS on it. On top of custom drivers which are not necessarily upstreamed, qualcomm socs always require closed source userspace daemons which are coupled to the kernel.
Valve have been working with Linaro to develop FOSS drivers for the Adreno 750. This is necessary, given how heavily Valve leans on having integrations with Mesa whereas Qualcomm's drivers are designed for an Android environment.
I don't see why they wouldn't unlock the bootloader, it wouldn't be the first Qualcomm-based product to allow it and in press interviews they have pressed, quite hard, that the Frame is still a PC.
