Optinal tags have always been allowed in HTML, for the simple if debatable reason (hence xhtml) that some humans still author documents by hand, knowingly skip md et al _and_ want to write as few characters as possible (I do!).

This is clear in Tim Berners-Lee's seminal, pre-Netscape "HTML Tags" document [0], through HTML 4 [4] and (as you point out) through the current living standard [5].

[0] https://www.w3.org/History/19921103-hypertext/hypertext/WWW/...

[4] https://www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.1

[5] https://html.spec.whatwg.org/multipage/syntax.html#optional-...