This is a flag to help others in the future.
If you install the Yoast SEO plugin, among the recommended settings is to “Disable unwanted content formats.” However, this section blocks RSS feeds. My emails stopped sending but searching the various sites found nothing. Then I found this on WordPress Yoast support:
https://wordpress.org/support/topic/rss-feeds-not-working-with-yoast-enabled/
Then the fix was obvious…