Google has upgraded the AMP enhancement report within Google Search Console to show Signed Exchange related issues. Google noted that this does not impact your rankings in Google search, that this is just a reporting update within Google Search Console.
This update was released on August 18th, so you may see additional notices in your AMP enhancement report today.
A Signed HTTP Exchange (or “SXG”) is a subset of the emerging technology called Web Packages, which enables publishers to safely make their content portable, i.e. available for redistribution by other parties, while still keeping the content’s integrity and attribution. Portable content has many benefits, from enabling faster content delivery to facilitating content sharing between users, and simpler offline experiences.
There are two ways to see if you have issues here:
(1) In the URL Inspection tool, click the issue under AMP version details.
(2) In the AMP Status report, click a URL in the issue details table.
Here are the current issues you can see:
For more details, see this help article.
Update: Google just posted about this over here with some more details and screen shots.
Here is the AMP enhancement report view of these issues:
Here is the URL Inspection view of these issues:
Forum discussion at Twitter.