About 15 years ago, aircraft safety systems at that point. One area where Waterfall is almost justifiable, but most of the other projects (in the same office) did not use Waterfall because (as that project demonstrated) it was crap. V-Model or Iterative & Incremental (slow-motion Scrum) were used with much greater success on all other projects. Success as in, they were rarely late by more than a rounding error (days, maybe a couple weeks). When they were late by larger margins the issues causing the delays were discovered early and were almost always requirements issues or hardware issues (that is, the former was partly on us, the latter never was).

I've seen Waterfall used on other projects since then including a major information system that resulted in about $1 billion of waste, fortunately I've only been adjacent to them not on them. The billion dollar waste relied on (as Waterfall often does) late integration, none of the pieces worked together even though everyone built everything per the spec they'd been given.

If you've not encountered it, that's great. Doesn't mean it doesn't exist.