Thanks for the report and sorry for the long delay!
The rpc.pingomatic.com subdomain still does not seem to support HTTPS, but I made a patch to update the schema for new sites if the subdomain gets SSL.
All unit tests passed with only the schema.php change. I also wanted to update the unit test to check both HTTP and HTTPS, and (only) the 7.0-multisite test failed. The failure notice mentions a timeout. (I had planned to try a pull request on my fork first. :facepalm:)
When this is completed, the Update Services support article will need updating (both the screenshot and the list).