[BobbyTables2]

Today they still boot in 16-bit real mode. Very quickly, it usually switches to 32bit mode for the BIOS, maybe eventually 64bit mode.

Option ROMs still start in real mode (at least for non-UEFI). The system management (SMI) handlers are still launched in real mode but too!

When the OS gets launched, l it brings up the other CPUs in real mode (INIT/SIPI), and has to do all the same gymnastics again…

Even PCI configuration space access is still supported with IO ports and that mode still required for some aspects.

Even today, the serial port still uses IO ports and interrupts just as it did 40+ years ago…

Yep, IO based POST codes are still a thing…