The original question was "how do you know these things before you buy the game?" My answer was "You could buy from a provider that advertises non-use of DRM like GOG." Whether it's a developer choice is irrelevant. GOG tells you the information you need for your purchasing decision, so if you want to know what you're buying, buy from somewhere like GOG. Also, don't assume that because it's DRM-free on GOG, it is also DRM-free elsewhere like Steam.

Buying a DRM-free copy on GOG seems like a perfectly reasonable thing to do even if a company has DRM on Steam; it provides an economic signal that there's some segment of customers that requires no DRM as a condition of sale. Since marginal cost of digital "goods" is ~0 and it's likely trivial to disable DRM in your build, it would be dumb not to cater to them and take your free money.