It means that variety has been removed from the collection, and SSE will no longer maintain it. If possible, SSE returns all stock of the abandoned variety to the original source. So unless someone else takes on stewardship, the variety will fade into extinction.

SSE's accessions policy has been tightening, to the point where it is difficult for them to accept anything new, regardless of usefulness or rarity. I hope I can garden long enough to see SSE adopt some of the varieties for which I am the original source.