Is there a page that documents what metadata is made available? From my understanding, it is very little, and designed with the intention that the servers could become decentralized. I doubt their company would make that move anytime soon, however.
Discussion
you can check the court orders that signal publishes.
iirc: timestamps, ip addresses, destination user phone number (not from sender afaik, that's encrypted)
So presumably with a unique IP, it would be possible to determine sender and receiver (however not likely in mass). If used with a VPN or Tor, it's much more difficult.