It is never good to wake up to a warning from Google.  Here is the text I received:

Search Console has identified that your site is affected by 1 AMP issues:

Top Errors

Errors can prevent your page or feature from appearing in Search results. The following errors were found on your site:

Custom JavaScript is not allowed.

We recommend that you fix these issues when possible to enable the best experience and coverage in Google Search.

This client receives much of his work through google searches.  I think google’s “recommendation” is more of a command.

I first updated all the plugins and the WP version, hoping that would clear the warning.  No, it was still there.  Next the most recent plugins were deactivated and the cache cleared. Still there.  Then almost all the plugins were deactivated and the warning was gone.  Gravity Forms is the plugin loading the illegal javascript in the AMP version.

The AMP plugin allows the suppression of plugins but supressing Gravity Forms did not remove its custom javascript.  I contacted the AMP plugin authors, who knew the Gravity Forms group was already working on this.  I logged into Gravity Forms and downloaded their latest version, which fixed the problem.