Starting August 4, 2021, a new certificate for server-based Game Center verification will be available via the publicKeyUrl. The previous certificate will no longer be available after this date. As a reminder, make sure your app always retrieves and uses the current publicKeyUrl value so it automatically uses the new certificate. If your app caches the certificate or hardcodes the certificate URL, it will require an update.
Please note, this root certificate issuer has been updated from Symantec Corporation to DigiCert, Inc. Make sure to check that the new root certificate issuer is on your list of trusted CAs. You can download the trusted root CA (DigiCert trusted G4) here.