Their “how it works” blog article is worth a read - they’re using a blackbox reverse engineering of the protocol (called PyPush) and re-implementing it natively in an android app, so there are no man-in-the-middle servers. It's pretty bonkers given how difficult Apple's spec-less tech can be to work with.
I’ve been using Beeper for almost two years now on desktop to view my iMessages, discord, and slack all in one app, and Apple hasn’t said or done shit about it yet
Per the Beeper CTO, evidently Beeper (cloud) isn't using the Mac minis anymore for anyone who migrated to their latest bridge. It's using the same solution, they just didn't make a big deal out of it at the time. I've actually been on the new bridge since early November and haven't had any issues. In my case the new bridge fixed an issue I was having with SMS forwarding from my iPhone.