The main issue is that there is no punishment for the ppl who spam d3 forum with d2 threads. Its left alone or moved. If it was deleted and the poster gets a penalty we would prolly see a drop in d2 posts in this forum.
Ppl just dont lose anything by posting d2 stuff in d3 forum.
They should start taking action towards these ppl as its simply is in the “Forum guidelines”. So there is no reason for the moderators to not delete those threads.