An example image of an alert can be seen on this page.
According to a blog post on TheSEMPost.com:
“Google sent out emails to affected webmasters, but it will also show up in Google Search Console messages for the affected sites.
There does seem to be a bit of a delay. So if you have actually upgraded in the last month or so, the warning might be outdated. And if you did a custom or selective upgrade, Google might still see evidence of the older version and send a warning.”