One thing that Steam/Valve has done with the Steam Deck is lock down the ISO by default, and provide no tools to modify your image persistently. That is of course on purpose, because that works for 99% of users, but the 1% of users may wanna use something where they can, for instance, overlay packages and keep them with updates, or apply extra gaming-focused tweaks that may be more of a hassle to maintain on SteamOS.
For instance, I use Fedora Silverblue daily on my Desktop, and even though it is immutable just like the Deck, it offers me tools to modify my image as I see fit and have the same modifications be applied to future updates too.
Bazzite is a Fedora Atomic-based operating system intended for Gaming, so you get a lot of the same benefits of that whilst still including all kinds of Gaming-Focused tweaks and patches out of the box. It is also immutable, e.g the system cannot be modified arbitrarily, whilst offering tools to still make modifications to the installed images if you want.