[admdly]

Fastmail have been singled out as they appear to have been given special treatment in the form of an APNS topic ID. Other hosts have been using a reverse engineered endpoint to generate certificates, which has recently been closed.

There’s some discussion on the Apple developer forums - https://developer.apple.com/forums/thread/778671. The solution for the OP there seems to have been they also will get special treatment, but there remains no route for others to use to get the same.