Another controversial absence is the "external link" symbol, which was rejected.

https://www.unicode.org/L2/L2006/06268-ext-link.pdf

The reason seems to be that links are not a feature of a text document, they are part of a markup language, and therefore, it should be a feature of that language instead.

For HN, AFAIK, they deliberately strip emoji, but most Unicode work fine. ("私はひどい下痢です" should show up correctly)