Before these T2 chips, Intel Macs were very close to regular PC hardware. That's why I suspected better Linux support on those older models. The list of still open issues on that wiki seems to confirm this.
IIRC T2 changed Macs from largely being IBM PC/EFI compatible to being closer to the architecture they now use on their ARM processors.
It deviates from the standard PC architecture, so generic drivers won't work anymore.
Since those machines were otherwise rather boring from a hardware perspective (rather expensive x86 machines that didn't even have the latest hardware) there wasn't much incentive to write those drivers so development is rather slow.
Yes. The T2 is in charge of a lot, keyboard/trackpad input on laptops, the SSD, etc. To get keyboard/trackpad input working you need a custom kernel as drivers still aren't in the mainline kernel.