This is the very same problem that Tor faced when Tor bridge use started to pick up in China around the late 2000s / early 2010s. You only needed a single Chine

This is the very same problem that Tor faced when Tor bridge use started to pick up in China around the late 2000s / early 2010s. You only needed a single Chinese user to connect to your server for it to be probed by the Chinese censors. Older versions of the obfs Tor bridge protocol could be detected by active probes and thus blocked very much like these Signal proxies. This is a cat and mouse game that Signal could very easily lose should Iran start to care about probing all new active connections that leave Iran.