For example, a “d-generate deck” consisting of nothing but mountains and lightning bolts. The term was adopted from mathematics where it means “mathematically simpler”, as in a d-generate parabola.

This was always my understanding of the term, as well, and it's the only context in which I continue to use the word. I casually understand "d-generate gameplay" as something that circumvents the game's mechanics, producing strong, simplified play, that's also significantly less fun / more annoying.