Mystery solved. Just ran into Akirad0e who is rewiring his tunnels a bit, and he blocked the passage by mistake. It's been restored.
Also... we discussed the possibility of setting up a Nether Bar (not to be confuse with a Leather Bar) in the beautiful scenic level half way down this stair case, and directly above the former Spawn Portal Room beside Admiral's farm. We could perhaps extend the platform a bit and add glass in the walls looking out over the visible Nether pathways threaded though the main Spawn area cavern. A community destination or meeting place worth visiting the Nether for.... and oh so convenient to get to.