It looks like Beeper has purchased the software from the person who developed this, but his GitHub repo for it is still up.
He has also created a way to permanently side load apps on iPhone without jailbreaking. I imagine Apple will be working on both of these “problems”.
iMessage reverse engineering hack that was purchased by Beeper: https://github.com/JJTech0130/pypush
iPhone side loading: https://github.com/SideStore
Beeper has been working in this space for awhile - there’s no way they would publish an app with a copyright violation. I assume they have a workaround for that aspect.