If necroposting isn't allowed to prevent spamming why aren't very old threads lock?
Answering something years after other member asked a question is stupid and clear case of necroposting.
Original topics doesn't have to be locked at all, unless original author decides he wants to do that, or if topic turns into spam fest that can result in moderator locking it.
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?
It depends from case to case, and there is no universal answer for this question.