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.

amd64 has had 48-bit addressing / 4-level paging from the beginning.

I must be thinking of intel’s failed 64 bit attempts prior to amd64 winning.

You're probably thinking of PAE (36 bit physical address space on 32 bit virtual address space). But Intel for quite some time had a 36 bit physical address space limit on 64-bit consumer processors, even if the virtual address space was 48 bits. I think it only changed 5 years ago or less.

Physical addresses may be limited to 40 bits, but 48-bit virtual addresses have been the norm since 4-level paging.