The first Usenet posts of Linus regarding Linux, or of Mike Godwin's coinage of Godwin's Law come to mind.

The question of how to determine cultural relevance is a hard one. Ordinarily, say, social utterances or discussions wouldn't qualify. If a participant or subject of that discussion ends up in a socially-significant role (high office, head of a major company, etc.), that might change.

Contexts vary, and can change over time.

At the same time, records are lost with time, there is a forgetting curve, and a few high-water marks stand out, though even those crumble in time.

Looking at HN's top-ever stories is an interesting exercise (either Lists or Algolia's search w/o any terms should give this).