I fixed it by creating /etc/modprobe.d/thinkpad_acpi.conf and putting this line in it:
options thinkpad_acpi hotkey=enable,0xfdffffThen you have to remove and reinstall the module to read the new options:
sudo rmmod thinkpad_acpiI'm not sure I'm getting all the brightness I should, but at least the control works to put the brightness up to make the screen readable.
sudo modprobe thinkpad_acpi
Whine: This isn't the first time some part of the interface between Ubuntu and my laptop has broken due to an update. I can sympathize with the developers and the difficulty of testing all laptops out there, but I can sympathize more with the user who isn't going to put up with an O/S that mysteriously starts and stops working as updates are installed.