I figured it out finally, and it is an ugly bug...
About 6 months ago, I set a location policy of US only. The policy included US locations that existed at that time, but I think the policy did not update with new US locations as they became available.
My GAE app was being deployed to "us-central1-f" (I did not specifically pick this location; I had previously picked us-central). This location was not in my location policy because I presume it did not exist then. I removed my location policy, and I can now deploy my app.
I never got a meaningful error message in deploying my own app. For a sanity check, I tried deploying Google's "hello world" GAE flex app, and that app gave an error message that included this:
> Location ZONE:us-central1-f violates constraint constraints/gcp.resourceLocations
So I was able to figure it out.
Could a Google Engineer please make an official bug report of this?