did you disabled SIP? I think this issue doesn't happen if have it disabled
SIP (system integrity protection; for those who don't know) was whatever it was. I neither activated/deactivated it. In any case, running post-install from USB restored keyboard/touchpad.
I still really do not understand why Apple pulled support for Macs that still can run current MacOS. Most people won't buy a new computer just to update OS. So the out of date software piles up and makes Apple look crummy and "fragmented". I know my MBP5,2 (2009) runs OS10.12 as well as or better than OS10.11.