We don't delete things that users have posted (unless they specifically ask us to).

It's long been a norm on HN that summary/tl;dr comments are not welcome. We want people's comments to be in response to the full article, not the summary. Sometimes a summary will be an inaccurate representation of the article, and when users base their response on the summary (without reading the article), it poisons the comments thread.

You weren't to know as it's not explicitly stated in the guidelines, but it is one of the norms that the moderation team and community has converged on over many years.