I can't get my Mac (10.13.6) to repeat letter keys when held: "fffffffffffffff" has to be typed out manually.
Delete and symbols repeat as expected and follow the keyboard control panel settings just fine.
I've also been googling around and found the toggle setting for ApplePressAndHoldEnabled and it is set to false:
[2019-03-07 10:01:30 /Users/Locane ]$ defaults read -g applepressandholdenabled 0
I've been through 2 reboots, restarted applications... I don't know what to try next.