It's not perfect yet, but it's much, much better than the old days.

OMEMO is supported by every major client, and they interoperate successfully. Unfortunately, most clients are stuck with an older version of the OMEMO spec. It's not ideal, but it doesn't cause any practical issue, unless you use Kaidan or UWPX, which only support the latest version.

All popular clients and servers support retrieving chat history now too.

In practice, I've been using it for several months to chat with friends and family, and haven't had any issues.

Perfect, now you just have to wrap your program inside a debugger in production!

Yep, that's the gist of it. In order to change the license from the GPL, they'd need the permission of all of the copyright holders who've contributed code under the GPL to the project. After a few months have passed, this basically makes it impossible (or at least extremely difficult) since at least one person (and likely many people) will say no.

They have also funded a lot of improvements to XMPP clients and servers.

I originally suggested Monal to my friend (who is quite into iOS and really appreciates a well designed application) and she found the same, but then she tried Siskin, and was happy enough to use it to this day.

Just for reference, here are my favourites on each platform.

Each support modern XMPP extensions, interoperate very nicely with each other, and (at least in my opinion) look good!

• Android: Conversations
• iOS: Siskin
• Windows & Linux: Dino
• Mac OS: Beagle IM
• Web: Converse JS

As a note of caution, I used Oracle's free tier to run a personal Matrix server, and it got deleted without any advance warning after a few months. I migrated to another provider and haven't had any issues for 2+ years now.

Along similar lines, I'd say Snikket. I feel XMPP often has quite a bad reputation based on the user experience from 10 years ago, but it's come such a long way and projects like Snikket make it very easy to get started.