At least since maybe the DEC Alpha 21264. It could address 48-bits of VA space, but that comes with caveats due to PALcode specific intricacies.

I think VMS (or was it Tru64?) uses this mode, but many other OSes just use 43-bit or 40-bit addressing. Realistically though, I don’t think many users would be using workloads that addressed more than 38-bits worth of contiguous VA space in 1998-1999.