I use “—⁂—” as my horizontal rule in plain text, via my Compose key (Compose+h+r).

HN removes things that are officially marked as emoji, normalises whitespace to space (which annoys me because I sometimes want to type things like narrow no-break space in equations which it makes wider and loses the no-breakyness of), and that’s all I’ve noticed. Control characters probably get some treatment or other too.