It was definitely the dominant conceptual approach, but I think it's an open question how much it was actually followed versus just being the lies everybody agreed to tell the executive caste.

One way I've weaned people off waterfall approaches is just to tell them yes, of course that's what we're doing, but to make it extra safe, we'll produce software early and often so you can see progress. Then eventually I'll slip in getting users to try it, just for "testing". And then getting it deployed early, for more "testing". The smart ones get engaged in this process, using it to making things better than the initial conceptions. They may produce waterfall documents for a while, especially if they need to pacify higher ups. But often they'll just forget. I think them GANTT charts and the like were always a ritual to get them feelings of control, and they stop doing that work once they have actual control.

So I suspect a lot of the nominal waterfall projects of yore were similarly made to work by developers just doing the right thing in quiet ways while saying, "Oh, that's 90% complete for sure. Mark it on your chart!"