Turns out that Adobe is collecting all of its customers' pictures into a machine learning training set.
This is opt-out, not opt-in so if you use Lightroom, for example, it defaults to adding all of your photos to the set.
If these are unpublished pictures, work-in-progress, etc. they'll still be analysed as soon as they're synced.
I've been using Lightroom to sync photos from my Windows desktop to my iPad. Now I need to reconsider that.
The kind of companies that opt you in something bad are the same kind of companies who change their preferences scheme from time to time, opting you in in whatever they like, with no warning.
The only way to win with these people is not to play.