As noted by Zac Hall, this kbase article about prepping your Mac for sale has a surprise step for those of us with Touch Bar Macs:
You can clear any information stored by the Touch Bar before you sell or give away your MacBook Pro.
First, start up from macOS Recovery: Hold down Command-R on your keyboard immediately after pressing the power button to turn on your Mac, or immediately after your Mac begins to restart.
When the macOS Utilities window appears, choose Utilities > Terminal in the menu bar. Type this command in Terminal:
xartutil --erase-allPress Return, type yes when asked if you’re sure, and then press Return again. Finally, choose Terminal > Quit Terminal and proceed to the next step.
When I first saw this, I assumed this has to do with the fingerprint information stored in the Secure Enclave. I rebooted into macOS Recovery, ran the command and rebooted again. After the restart, my Touch ID information had been wiped from the machine:
Mystery solved. Add this to the long list of things to get a Mac ready for sale that macOS should handle itself.