Apart from that, regardless if you're on Signal or Telegram if authorities get hold of a protester's identity on such an app and have the power to access the app's servers they can gradually uncover social networks by reading metadata (if I'm not mistaken).
I think you are mistaken. Before your text is sent to Signal your sender information is encrypted with the receiver's public key. So while Signal's servers can see who to deliver the message to they cannot see who sent it. Only the receiving client can decrypt and authenticate the message. This feature was rolled out in late 2018 and is called "sealed sender". It was developed to prevent leakage of any social network information via the message metadata.
But as far as I know Telegram has no equivalent feature.
"So while Signal's servers can see who to deliver the message to they cannot see who sent it."
Why can't they look at the TCP headers of incoming packets to determine source-IP? Also, why can't they look at session identifier or signal ID like phone number to determine who the sender is?
I assume if you are trying to hide your communications you aren't connecting directly to signals servers, so IP should get you nothing. There is no session identifier or signalID attached to your message, its contained within the encrypted part of the message so only the receiver can determine who the message was sent by. https://signal.org/blog/sealed-sender/
