Is he gone for good?
The Culber we saw task Stamets with saving the mycellium is I think an agent of the Mycellium itself. I think the network has a consciousness and has manifested itself as Culber as this would be the form that Stamets would be most receptive to. It's possible that he will continue to appear to Stamets as he tries to solve the problem caused by mirror Stamets. This may mean that he remains in the programme for quite a while yet which would be good it also allows the possibility at some point of Stamets being absorbed into the network to be reunited with his partner. I think also that the network forms some sort of framework to the universes and if it fails then then the universes will also fail.