From this card:

• Leverage WordPress post embeds for embedding AMP stories into posts. Make sure Post Embed block can embed AMP Story post type. Use featured image, title, and excerpt in preview. (Converted to Leverage WordPress post embeds for embedding AMP stories into posts #1931)
• Set fallback default image in amp_story template when none is provided so that validation of the story is preserved. The warning should remain.
• Show warning notice under the featured image control when no image is selected. Remove the global notice.
• Extend Gutenberg pre-publish panel to block publishing while featured image is not set.

From this card:

• Determine if "automatically" is the correct approach
  Either or...
• Background media assigned to the first page to automatically populate featured image, if empty
• Instead of automatically being set in the postmeta, add a get_post_metadata filter for _thumbnail_id to supply image if it will be empty