Most people know nothing about "Waterfall Model" which if you think about it, is just commonsense meta framework using which you can implement any SDLC methodology. Pair it with "Spiral Model" (https://en.wikipedia.org/wiki/Spiral_model) and you are golden.

This excellent writeup by David Olson gives both the history and the correct understanding; The Myth of the 'Waterfall' SDLC - http://www.bawiki.com/wiki/Waterfall.html

Reading this subthread is wild. I worked for NASA earlier in my career. We never used the term waterfall, but the process described by that term is epitomized by large government projects, and NASA engineering culture in particular.

Most of what people are describing here would be unrecognizable to any government program office.