Signal Snap Maintainer here, this is because of a DMCA takedown request from lawyers representing Signal. Canonical is currently working with them to clear things up. Canonical's communication to me was initially lacking due to issues in their process, the process has been amended and I'm back in the loop again.
This deserves a lot of publicity. Why send a DMCA takedown instead of contact the people involved? Will they do the same if some vulnerability is found in Signal? Lawyer up instead of fix the problem?
No, those lawyers are paid for by and directed by signal. Signal is responsible.