All of the ones I participate on specifically don’t lock old threads, and for specific discussions, it’s expected that instead of making a new post on the Prophet 12, you look up the old one and add your post to it. If found forums that lock old thread to be very annoying, and I generally don’t use them for that reason. Discussions can easily spread over a decade, and that mega thread is a wealth of information.

But it doesn't happen as fast as it does on HN.

Once something drops from HN's front page - 12 hours? 24 hours? 36 hours if it is really huge

then it's dead.

but they need to have died for that. I know plenty forums that have threads that are running for months or years - because they've never gone for $threshold weeks without a post. (and in general I tend to think locking after time is bad anyways, or at least should be able to be overridden)

Whereas even within the days where it is still possible, you are not going to see "oh, this HN/Reddit discussion from last week is still active" because it doesnt get ranked anywhere based on that.