Isn't SteamOS immutable? Can you layer packages on it like you can with Fedora Silverblue?

You can add packages, but they can be wiped by the updates. Flatpaks work seamlessly and because of the Deck's popularity, most everything you would want is available in flatpak form

On the Steam Deck you boot into desktop mode and it’s a standard Linux. Install what you want. I have Heroic Launcher on mine, running games from GOG and Epic alongside Steam games.