That hasn’t been true for like ten years now.
Obviously anything that needs an internet connection will require steam. But pretty much almost all single player games do not need steam to run.
No you dont? You can literally just copy files onto an external hard drive, there is no requirement to use steam to do that.
It varies a bit from game to game, but typically Steam games are intertwined with Steam in one way or another. You can move the files around, but you need Steam to verify and “fix” the files and their associations afterwards.
It definitely does vary though. For example, with KSP I was able to just copy the install directory and have many different install folders for different instances of the game (great for version and mod control). For others, I was able to copy the files, but it didn’t run, not until I manually set up Steam to the install directory and did the verify integrity thing.
