The good news is that my keyboard started working again after the next update! So the problem resolved itself. Until your next update, perhaps you can try running your keyboard through a USB hub, that worked for me. Or if you have another non Apple keyboard, try that.
As for computers slowing down after updates, I feel your pain. My 2015 MacBook Pro was getting really bogged down last month after a couple of updates, but has improved a bit since then. Sometimes the slow down is caused by programs updating in the background. Photos is notorious for taking days to update its database in the background and can cause everything else to grind to a halt. And the inevitable march of progress means that as operating systems continue to get more competent and more complicated, they "outgrow" old hardware. I really don't think it's planned obsolescence, just over excited computer engineers forgetting the rest of us don't have the latest Mac Pro.
Funny, this post would have appeared days earlier, but Apple's support forums failed!