From gbhackers.com
GitHub announced that it suffered a security breach in which unauthorized individuals obtained access to specific development and release planning repositories and stole encrypted code-signing certificates for the Desktop and Atom applications.
Hence, in order to avoid any potential misunderstandings, the company has made the decision to revoke the certificates exposed to public scrutiny.
There will be a limitation to the functionality of GitHub Desktop for Mac and Atom when these certificates are revoked.