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... USB booting has been a thing since almost as long as the USB standard has existed. You don't need UEFI to boot via USB. Whoever you found that said otherwise doesn't know anything about PC hardware. The earliest boards I've seen with USB boot support were way back in the later 90s, like Super Socket 7 boards and Slot 1 Pentium 3s. By the early 2000s, USB boot support was pretty widespread.
Even for BIOSes that didn't explicitly supporting from USB had the option to use a special boot loader like PLOP to boot from USB. Said people that said USB booting was not possible on older BIOS machines were probably confused because bootable USB devices on BIOS machines was often shown as a hard drive. You had to select hard drive or USB hard drive, and then go into the hard drive boot order and put the USB device at the top of the list. Sometimes they were also shown as USB FDD, USB ARMD or USB CDROM, it really depended on the BIOS vendor and board vendor to how it worked, there was no unified standard.
While PCIe is supposed to be backwards compatible by design, early PCIe implementations had problems with newer PCIe cards. There was a big fiasco when PCIe 2.0 came out where motherboards with 1.0a and 1.1 slots would fail to boot with a PCIe 2.0 card installed. The fix was a BIOS update, but this was at the discretion of motherboard vendors and many motherboards never got the required update, forcing them to be stuck with older PCIe 1.x cards. I've run across this issue numerous times over the years, including on OEM systems from Dell and HP which never got the required BIOS update and were stuck with older PCIe cards. The boards that did receive an update usually worked fine afterwards and even worked fine with newer PCIe 3.0 cards as well that still had BIOS support.
There were boards with "AGP" in the 939 era, but many were fake and not really AGP slots. There weren't many 939 chipsets that officially supported AGP, so motherboard manufacturers jimmy rigged proprietary solutions that had varying rates of compatibility and performance. The lowest garbage solutions often just connected an AGP slot to the PCI bus.
https://en.wikipedia.org/wiki/Accelerated_Graphics_Port#Unofficial_variations
