I ran across WebXDC this past weekend, and I'm intrigued. I've built a few apps on it already, and I'm very pleasantly surprised by how cleverly inverted the whole model is (in combination with ChatMail). Delta Chat is available on iOS, I believe. I use Arcanechat on Android.

https://webxdc.org/

https://chatmail.at/

https://apps.apple.com/us/app/delta-chat/id1459523234

https://arcanechat.me/

The beauty of the ecosystem is that apps are just zips of a webapp that is self-contained, and uses the webxdc api to message other copies of itself running on other folks' phones that are part of that chat group. Basically, the app communicates (and is distributed over!) the group chat (it's otherwise insulated from the internet). And because apps are just zipped webapps, they can be pulled apart, hacked on, and reassembled as needed for your group.

ChatMail relays tend not to retain info - they aren't hosts, they just relay encrypted messages (the platform mandates encryption). This sidesteps all the approval/review processes, and there is no app store or developer fee or even SDK. It's just folks trading apps and running them together.

It can't solve a lot of cases, but it solves some use cases (like a shared grocery list, or real-time card games, or games with leaderboards) really well.