Certificate validation is incredibly important for a secure internet!
However, what happens if a certificate gets revoked? Well, the certificate, being validated, must be ALSO be checked against a list of known bad certificates. The problem is that this is a hard problem to solve.
The Online Certificate Status Protocol (OCSP) is a way that allows to find a certificate is has been revoked. Apple has this setup to check for the revoked certificate.
Why did MacOS user applications stop working? The OSCP endpoint was OVERLOADED with traffic. Because of this, the validation of the certificate could not been done, resulting in applications not running.
What was the temporary solution? To redirect the hosts file to a different IP (that is not on). Applications must work if offline so the request ONLY happens when the computer is online.