People are right about flatpak - it will generally keep stuff out of your actual root/home directory. But like you implied, the steam flatpak is unofficial so you may run into issues. With that said, I’ve used it and know many people who use it without any problems.
And depending on the game, you might be able to run it directly with steam offline, or even straight from the executable without steam open at all.
Of course this isn’t airtight, but there are ways to check the permissions granted to flatpak applications. And IMO it works well enough for games. Ofc this depends on how paranoid you are and your reasons for wanting this (fear of a game being a virus, not wanting clutter in home, wanting protection from a bug that would delete data, etc.).
May I introduce you to ﷽