There used to exist applications like this, at the time (i.e. the 90s) when apps didn't have automatic updates. What happens now is that the same company that runs the server and makes the official app updates them both at the same time to change the protocol, causing the app that supports all of them to break until they update it to support the new secret protocol. Then people stop using it because it's breaking all the time.

Social media aggregators intended to post messages to many clients have been tried before, but they have consistently failed because the social media companies, notably Twitter, changed their APIs to frustrate the possibility.

Seems similar to spread spectrum radio signals, also known as frequency-hopping. By hopping from frequency to frequency in a way that only the sender and the receiver know, you can avoid both obstacles and eavesdropping.