I think people would be more active on this forum if they could subscribe to replies. To avoid getting flooded by this, just send one e-mail per thread (saying that there may be more replies), until the thread has been viewed using the link in the e-mail and reactivated the thread's subscription.

Subscribe:
Set state to active

Unsubscribe:
Set state to off

Update:
If active then send e-mail and set to passive

Read mail:
If passive then set state to active

This is why is use RSS feed: https://handmade.network/atom
It's perfect for this kind of use case.

This is on our list to do someday, but we do indeed recommend the RSS feed (or just the feed page) as the main way to keep up with the latest activity!