The problem with push notifications is that they need to go through the app provider and incur costs for it, that's not really their fault. If they'd not charge for it, they'd still go through their servers and would lose them money. So putting it behind a paid service you hook up to your self-hosted instance seems fair.
If you want to avoid it you'd need to build patched versions of the app and distribute them yourself to your users, so you pay Google/Apple directly for notifications instead of going through Zulip.
Curiously if you ask if they would consider properly supporting notifications from the mobile web version then the answer is always no. Pwa would be perfectly fine but its crippled on Zulip
If you read their announcement for the new paid subscription they do not hide the fact that this is first and foremost designed as a revenue source. One could potentially fork the clients and add such functionality. But they will not do it on their own clients.
If you want to avoid it you'd need to build patched versions of the app and distribute them yourself to your users, so you pay Google/Apple directly for notifications instead of going through Zulip.