1. If necroposting isn't allowed to prevent spamming why aren't very old threads lock?
Old threads should be lock especially when the discussion starting to be nonsense and all replies are generic answers or already out on the topic. But if the thread still helpful since the thread creator open it to leave an open discussion towards certain issue then I guess its still good to let it open.
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?
If you can contribute to the topic post it there since its nonsense to create new thread since there is a topic already posted intended for that.