Serious question since I don't use iMessage whatsoever, what's going on with the iMessage stuff? Seems like multiple companies recently have tried to make apps that connect to iMessage, but there's nothing I've heard about Apple opening that up. Did something happen for this to suddenly pop up more frequently?
Someone (possibly recently?) figured out the protocol and how to register a phone number without needing an apple device. Older versions of stuff like this required having a Mac virtual machine and routing messages through it using a user's AppleID, so this was much easier. I saw a video that was bragging about how this new method would be very difficult to block because doing so could affect regular users, and I just kinda laughed at the naivety.
In the US it's the messaging standard because they are too lazy to install a cross platform messenger like everybody else in the world. So Android has a 40% market share there, which is the minority but not a crushing minority like Windows–Linux but for whatever reason American society rather focuses on iMessage than just to install Signal or whatever.
Pretty much it's the Beeper devs and one other. But the initial setups were really nothing more than using a Mac on the backend with a an adapter to Android.
Beeper and one (maybe two) other were pretty effective at it.
Beeper Mini is a different thing altogether. It uses a service to translate ANP (Apple Notification Protocol?) to GCM (Google Cloud Messaging), which are the respective notification handlers.
The Android client is able to comm directly with iMessage servers, unlike the original Beeper and the other ones.