That doesn't mean you can't control how it works. Most people don't need sources of their Linux distros to use them as they want. It would be cool to have the source, but you wouldn't expect them to have an official maintained repo since they spend much more resources on actual hardware that needs this distro.
Steam client for sure is proprietary and it comes with the OS
Yeah it seems to also be the only thing that is proprietary in SteamOS too.
To play any game you have to install and run the proprietary Steam client and be logged in to an account.
Are you clueless or what? There are too many ways to do what you want with SteamOS. You can use offline mode, desktop mode, play pirated games in any mode, install any controller software you like. Finally, install another Linux distro on it, or Windows. But people buy Deck because of SteamOS mostly since it creates the intended (and expected) experience.
Wanna know why we aren't seeing many enthusiasts creating more handheld frontends for platforms like Deck? Yeah, not at all because the platform is locked behind DRM or other bs. But because the best experience most people expect is already available and it becomes better with updates.