Responding to a thread that has been lacking activity for quite a while (month or even years) is a context based decision. If reading through you find that you can add something meaningful, then it makes sense to add to the thread (i.e. adding further developments on a given topic being treated, or genuine related questions). One has to calibre properly if it makes sense or not-

Generally speaking, people don’t go bumping oldish threads that much, and if they do, other will notice clearly whether the intent was to say something meaningful or not (and likely subsequently report the post if not).

In the big picture, people don’t really resort much to locking threads themselves. I believe I might have locked a few in Meta when I asked something and got satisfactory replies, and a few time-sensitive threads that made sense to lock, but besides that, the others remain open primarily in case someone needs to add or ask something further down the road, alongside there not being a collective habit of locking them.

One would think that threads that have not been active for a certain amount of time could self-lock, giving the OP the option to reopen them. A mechanism could be devised whereby threads could self-lock by default after x days without any activity (i.e. 90), giving the OP the option to adjust or even remove the default value x.
Nevertheless, I’m sure we could find plenty of counter-arguments to vouch for this not to take place (i.e. can’t add more important related info nor ask valid related questions, it calls for opening new threads rather than building-up further context in an already existing ones, and so forth).