I have been stoked the last couple of days and caught up with my email. Then I found this:
You are receiving this email because you have one active Let's Encrypt certificate requested via DNSimple and you may be affected by this change.
It has come to our attention that we are currently including by default, in our HTTPS redirector and DNSimple app certificate installer, an intermediate SSL certificate for Let's Encrypt that will expire on September 30, 2021.
This was sent 2 days ago... "It has come to our attention...", well that explains it but they should have known and tested ages ago. What to say?