As far as I know there is no mandatory DRM on Steam either, so if a publisher wants to they can just make their game be portable and not require Steam to even be installed. Pretty sure all the re-releases that use DOSBox or ScummVM are like this, for example.
lud@lemm.ee 5 weeks ago
Yeah there are loads of DRM free games on steam (mostly indies of course). Steam just offers a very basic (and easily bypassable if you know how) DRM to debs/publishers but they absolutely don’t need to use it.