I have myself necroposted recently and frankly, I hesitated between helping a few people that would find the page via search engine, or try hard not to bother people looking at the top of the forum section (I suppose it only concerns people who are enabling notifications for the whole forum? I did that for UI once and then gave up on it, there were too many threads that didn’t concern me).
I was suggesting to add an opt-out checkbox “Do not bump to top of threads (check this when adding minor information to old threads)” in the post options; or possibly the opt-in equivalent.
It would be convenient for minor comments that are worth saying to people watching the thread / finding the page via search engine, but not worth saying to everybody else.
Problem is, options are currently hidden in More Options… which some people may miss.
If the checkbox is opt-in (Bump), then most people will miss it and most posts won’t bump at all.
If the checkbox is opt-out (Do not bump), then most people will miss it and necro will bump as usual.
So it may be a good idea to always show the options (as in other forums) in combination with this (we should anyway, some people may forget to subscribe to new posts because of this).
We could combine this with the warning system: if thread is old, ask confirmation to bump (opt-in is checked, or more importantly if opt-out is unchecked).
Finally, we could combine this with the Resolved status suggested in another thread: Forum Feature Request: Mark a thread as resolved
If thread is resolved, the checkbox is set not to bump by default (you could argue that forum threads are for open discussions only and Unity Answers for closed discussions, and I would agree, but sometimes we need a thread to post answers in a longer format, esp. when sharing big code snippets).
Realistically though, it seems that Unity Forum is using some templated solution (somebody mentioned they’d be looking at a plugin for thread auto-lock for instance), so not sure how much they can modify precise behavior…