From my understanding, Mihoyo accidentally (?) released the entire 1.5 beta client a few days ago, which is what led to the initial avalanche of 1.5 leaks. After Paimon's twitter announcement, they then accidentally (?) released the whole ass 1.5 developer build (jesus...). The same leakers who initially leaked the entirety of 1.5 have also been sharing the development kit around and putting up public mirrors of it. This is obviously a huge fuck up on Mihoyo's part and probably why they shut down the beta server. Lumie has stated that they deactivated in part because they did not want to be associated with the sharing of the beta client.
To clarify: the 1.5 leaks are not coming from beta testers who have signed NDAs. Mihoyo essentially made it public themselves, and if you understand how/where to look, you can find it.