OK; I upgraded mChat, OneAll Social Login, and Stop Forum Spam, all of which have tested with 3.2. I know that Site Logo is still RC6, but I've been running it forever and it works great - and it's designed for 3.2. The warning persisted.
I then disabled my board, backed up my database, disabled the extensions, and deleted all their data:
The phpbb_ext table is now empty!
...but: The warning persists -
Unless someone can suggest something less invasive to diagnose, I'm going to wait until 3.2.3 in the hopes the upgrade will make this issue disappear. It's simply too traumatic to go through the machinations of replacing the database. Yeah; I give up. Thanks for all who tried to help.