Withers can't despawn. They can just fly away. Also, a wither despawning doesn't make you lose diamond stuff in the slightest. Only death would cause such a thing, which items are not refunded for. In fact, items are generally not refunded by the staff at all.
about 3 hours ago wilderness:-170,48,-647 - barney0110 was doomed to fall by Wither You were killed by the wither and it despawned when you died because no one was in the area. The items are lost forever sorry for lost.