Can you backup steam games and run them without steam?

Many of them, yes. It's up to the individual publisher whether they want to release their game DRM-free on steam. https://www.pcgamingwiki.com/wiki/The_big_list_of_DRM-free_g...

If you want to guarantee that you can backup your PC games and run them without a client, then there is always https://www.gog.com/