What If: Signal Was Part of the Fediverse?

That's fair! If you're on these type of forums, there are a lot of Signal haters and a lot of Matrix lovers, and sometimes they like to make confusing or just straight up inaccurate statements. The crux of the issue is not about the encryption of the text of messages themselves, which both platforms are capable of doing. Personally, I wish there was something like Signal but without the centralization, but the reality is such a thing doesn't exist.

Signal (as in the Signal server and by extension the legal entity behind Signal) does not know what groups you're in, does not know who's in your contact list, does not know which groups you are sending messages to, doesn't know which groups exist, and can't tell the difference between a message, a reaction, a read receipt, a remote delete ("delete for everyone"), an edit... etc. Signal doesn't have a way to send anything between two parties that the server can see. Signal has received a number of subpoenas which they typically fight, and if/when they lose they over all of the information they have about the subject of the subpoena, which tends to be whether or not they have a Signal account, when they registered the account and when they last used it. You can see these at https://signal.org/bigbrother/

Matrix (as in the Matrix server you're registered on as well as the servers of whoever you're talking to, for groups that means everyone in the group, notably this is not necessarily the same as the legal entity behind Matrix, but in practice a LOT of people use matrix.org for their home server so it frequently is) can see basically all of the things I listed above. The text of normal messages is encrypted. The group membership list isn't encrypted. reactions aren't encrypted. read receipts aren't encrypted. Group membership lists are stored in plain text.