Apple's complaint will likely be that it's a "technical limitation" because they run the only iOS notification gateway.
Never mind that Mozilla manages to run one for Firefox completely free to users and devs despite being a comically mismanaged nonprofit and if it were really a problem for them they could allow users to enter the domain name for an alternative one.
This issue right here is actually why there have been so few usable open source federated chat apps on the iPhone: the client maintainers must also maintain infrastructure for notifications and are not allowed to delegate this to people hosting their own infrastructure. This is actually the core complaint many people have with how Apple runs their app store and it's very visibly made the internet less usable for everyone.