I think that a useful feature in this forum could be the ability to intentionally respond to threads without sending them to the front page and without notifying respondents.

Such feature already exists in image boards like 2chan already implement this feature since long ago. It can add incentive to discuss older threads without the fear of 'necro bumping' and also when users want to respond to a thread critically without promoting it via bumps.

Maybe this has been suggested already, but I couldn't find such. Personally, I've been advocating for reducing bumping right for new accounts since last year, I change which I see was implemented September 2019.

Now I think it'd be nice to allow for users to choose if they'll bump a thread.

Edit: after the discussion that has followed my thread, I would like to clarify that it wasn't my intention to make it so replies to threads could be made in complete 'silence'. IMO, a feature like sage in a forum would translate to either:
a. Responding without bringing the post to the front-page for everyone, limiting notifications only to past responders and followers.

AND/OR

b. Potentially limiting the bump. F.e.: Limited bump power applied & only bringing it to the front of the board at most, but not the entire forum.

This would be good for encouraging responses to old threads and also not giving credit to OPs that might not be worthy. For example, if you feel like quoting a response in a thread where the OP isn't a quality post, you might be deterred by the quality of the entire thread and would rather avoid bumping it as a whole. Experienced members could utilize such feature to also respond to unworthy threads critically. It would give bitcointalk users a choice on how much they'd want to utilize their bumping powers, and to which posts.