And 9x can still be virtualized even on the latest Intel iMac, with nested paging enabled (or not). On AMD, you need to disable nested paging due to the above change in behavior.
The Intel Macs were a strange not-quite-PC-compatible; the BIOS was EFI, and I believe things like the 8042 (which controls A20) weren't present although I'd have to check the schematic to be sure. That said, I'd be surprised if port 92h "fast A20" wasn't present --- that's been in nearly every chipset since ~386 or so.
And 9x can still be virtualized even on the latest Intel iMac, with nested paging enabled (or not). On AMD, you need to disable nested paging due to the above change in behavior.