Linux

TL;DR: Install Plasma/KDE on your current Linux and select it when logging in.

First a correction, SteamOS used to be debian based, it's now Arch based, not that that should matter to you because 90% of using a Linux for day to day will be through the DE or with commands that are the same for all distros, so anything with Plasma/KDE will look and behave the same as SteamOS. The remaining 10% has to do with how you install things, on SteamOS you don't install things to your system (i.e. the equivalent to apt/yum/pacman/portage in other distros) because it's immutable, but there is a store to install Flatpaks for your user which I'm sure you can install on other distros (or something similar enough).

So realistically any distro with KDE/Plasma should feel the same as SteamOS for you. While technically Arch/Manjaro/Garuda are the closest ones I wouldn't recommend any of them because Arch is bleeding edge, which means sometimes things will break, it's rare but it happens, and Arch's philosophy is that you should know how to fix it. If you already have Pop maybe just install Plasma on it and see how you like it, you can select your DE on the login screen, and you can even change the login manager program in the future if you want, and most of the things you'll learn by doing so will be usable on other Linux distros. Until you're VERY familiar with Linux the difference between a distro and another is just the package manager they use and what comes pre-installed by default, so use whatever you're already using unless you have a specific reason for wanting to change.