2002 iMac running Mojave with Apple wireless keyboard
Today, a cluster of characters on the right side of the keyboard (7 8 9 u i o j k l m .) quit working, both caps and lowercase.
To narrow down the problem, I replaced the keyboard with a spare and re-booted. The issue remained, leading me to believe it's not a keyboard problem.
I enabled the keyboard viewer and all keys work when clicked.
SOLVED — The problem was inadvertently enabling Mouse Keys.