You can always hit the 'threads' link in the navbar. Internet forums evolved without this glaring gap in basic functionality and some somehow continue to eke out an existence without it.
Internet forums have had the ability to get email notifications when there was a reply to subscribed posts, username mentions, and private messages for 20+ years.
Some internet forums, lots of others have done without. Either way, the idea you can't have a forum without it isn't quite right given that HN has outlasted most.