1. If necroposting isn't allowed to prevent spamming why aren't very old threads lock?
I can't see any rule that prohibited necroposting. [...]
Regarding necro posting, if the necro post is substantial (doesn't break the rule especially rule 1 and 2 (2014/07/21 version)), then it's OK
Please be aware that, if the necro post is
not substantial, then it's not OK anymore (at least, this is what I understand from mprep's statement).
So, basically, there is no rule against necro posting. However, if OP wishes to bring back to life an old thread, this can be done, in my opinion, by editing it (if he was the author + combined with a post like "March 2023 Update: I found new evidence / new information etc.") or adding new valuable information to what has been discussed, if he believes that it would be useless to start another thread on same matter, since one was already created. Take for example the topic
Merry Christmas to the Forum, which is brought back to life each year on Christmas. Practically, by typing there after 365 days, those posts can be considered as necro posts. Yet there is no reason for starting a new similar thread each year.
2. If I come across an old thread from 2021 and I think I have new input to contribute, would do you advice me to start a new thread instead of bumping the old one?
Same as above. Each case should be analyzed on an individual basis. If what you want to add is precisely on the subject of the topic, then you could add your post to the old thread.
Basically these guidelines can be considered as
common sense. If you feel that you are not on same page with the main subject, then start a new thread. But if you'll discuss precisely same ideas, you can use the old topic.