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.

Steam is its own DRM on top of whatever else a developer chooses to do. I found this out one year when I spent months without internet access. At a certain point steam would refuse to run any of the locally installed single player games I had paid for through their platform until my computer phoned home to their servers. I'd already configured everything for working offline and they did successfully for a long time until one day they just wouldn't anymore.

If you don't want lose access to every game you fully paid for on Steam you'd better pirate a copy of everything you bought because on a whim they can take it all from you at any time.

I'm honestly pretty disappointed that GOG is still selling the game. If they are going to sell it at all they should have massive warnings all over the page that the game is broken. https://www.gog.com/en/game/cult_of_the_lamb