Warning are not handed out automagically, but by moderators.
If you feel a warning was issued incorrectly click the appeal button and explain why and an Admin will look further into it.
On a side note, if you make a post then realise right away its say a necro post, you could report the post yourself and explain that you just realised and would like the post removed due to it being bad.
It has been suggested before about an auto pop up to remind someone the thread is old, but if your post is continuing the discussion in a meaningful way and not just reviving the thread for no reason then its not against the rules, so a warning pop up on an old thread may not work, but thats down to Curse to the higher ups to decide
I think it was issued correctly, but my point is, it would be unnecessary to even issue the warning if I had been told the thread was necroed at the very moment I click on reply or quote. I didnt even see the date of the last post on my hastiness.
Also, it is not intended to help who is trying to add important information to the thread, they will determine if what they have to say is important enough or not to revive it. It is intended to help ppl that didnt realise it was necroed...
Also, I wonder if the post removal would also put the thread back away from the newest 1st threads page, I believe it wouldnt work but only you that code it can say that