I really want to use zulip but last I checked they charged for push notifications. Without it, my group lost interest in using it due to missed messages.
I saw a post about webpush a day ago. Not sure if anything has changed for zulip in that area.
We charge businesses for our push notifications service because we need folks using our 100% open-source product to run their business to help pay the cost of developing it.
https://github.com/zulip/zulip/blob/9.4/LICENSE (Apache 2) and if you mean "good [hosted] replacement" https://zulip.com/for/open-source/
It, of course, does not speak to your decentralized wishlist but I'm sure they'd welcome an issue describing your goals