Can we still use .p12 certificates with the HTTP/2 protocol instead of TLS or do we need to use the new .p8 method of authentication?
.p12 Certificate with HTTP/2 protocol
Yes, you can still use a certificate based authentication with the HTTP/2 protocol as described here: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/establishing_a_certificate-based_connection_to_apns