You will have turned off the function "use smart quotes and dashes" in the spelling & prediction settings.

No, it's on (by default I assume).

In certain places it does seem to do the substitution - Notes for example - but in comment boxes on here and (old) Reddit at least it doesn't.

It’s a per-app setting that sometimes needs to be set in the text field’s context menu. There’s also a few apps that just don’t integrate with the macOS text system.

Well, I see no way to set it in Chrome. A good reason to be a little suspicious of Reddit or HN posts with em-dashes IMO.

It does on Safari (Mac and iOS) by default on Reddit.

Probably under Edit → Substitutions. That’s where it usually is in apps.

A long press of - should give it as an option as well. Mine auto translates it but I can’t recall if I added it or not.