Based on the prices I see them selling for via findUO, I'm guessing the ones with +necro still spawn, they're just much rarer. If it no longer spawned with +necro, I'd expect to see:

1) Vendor descriptions that include, "OMG no longer spawns"
2) Much higher prices than 100-200k normal, 1.5M for "owned by no one"

Can anyone confirm?

~M