Some games can be played without client (you just go into steamapps directory and run them from there) and it should be possible to use fake client as well, so backing up your Steam games is very possible.
https://steam.fandom.com/wiki/List_of_DRM-free_games
edit: Interesting – looking at this list I found out that Dust: An Elysian Tail can be played DRM-free on Linux, but not on Windows.