Not all Superchargers are showing this yet, but select Superchargers started showing live stall availability to help with trip planning/congestion issues. Presumably, they've prioritized the busiest chargers first. It seems like the capability was added in the last general release version (8.0 (2.52.22)) which started showed the cost for idle fees on update but not stall use. Now this additional data is turning up. It wasn’t there before for the same charger and my car firmware version hasn’t changed.