Why, it works quite well. In short, the scheme is this:
XMPP server -> Push service -> APNS / FCM -> XMPP client.
Push service here is a service maintained by an XMPP client developers, cause you need a client app certificate to be able to send messages to it via APNS/FCM, and no sane devs will be handing out their certificates to third parties.
It should be noted, that the recent changes to how Apple push notifications work in iOS 13 (MORE restrictions on VoIP and 'silent' background notifications) require some updates to it, but so far, XEP-0357 does not look like something terrible.
XMPP server -> Push service -> APNS / FCM -> XMPP client.
Push service here is a service maintained by an XMPP client developers, cause you need a client app certificate to be able to send messages to it via APNS/FCM, and no sane devs will be handing out their certificates to third parties.
It should be noted, that the recent changes to how Apple push notifications work in iOS 13 (MORE restrictions on VoIP and 'silent' background notifications) require some updates to it, but so far, XEP-0357 does not look like something terrible.