Using Damus on iOS. The most important thing is push notifications, but I’m not sure how this can be implemented in a decentralized encrypted way. Maybe the push should not contain the content of the message?
Second required feature is chat groups.
Discussion
1. When you say push notifications, do you mean "when a new message to you is created, you get a notification from the app?"
I believe this has been implemented by Damus. Blowater has it as well.
2. Yes, encrypted chat groups are very important.