There are 58 prior HN comments with plantuml.com, mostly in various discussions of diagramming tools, but not only that: https://ampie.app/url-context?url=plantuml.com And a bunch of pages on the same domain have also been mentioned in HN comments.

There was also a thread about Plant UML in 2019 with 48 comments: https://news.ycombinator.com/item?id=21426793