But is there a way to actually back up an installer (or the installed game) in a way that lets you run it on a different computer after, say, your Steam account has been closed for whatever reason?
On macOS an app bundle is usually self-contained, so if there is no DRM backups are simple enough; for Windows, it's usually not that easy, I believe.
For some games the executable just works outside of Steam, you can copy the whole directory from steamapps to elsewhere. I recall Starbound being like this during beta, good for last-minute local multiplayer at a LAN party.
Those games aside, I think goldberg steam emulator is used for ripping/cracking/sharing games. I don't wanna say too much as I don't know the process well, but I think sometimes you remove or replace a certain steam file and that's the main thing needed. This doesn't get you past extra layers of DRM like Denuvo, of course. I recommend not buying any such games. You can follow a Steam curator that checks games for Denuvo and then it will display if they have it (or used to and removed it, like Soulcalibur VI) on the store page if you're logged in.
