You are conflating the concept and the implementation. PFS is a feature of network protocols, and they are a frequently cited example, but they are not part of the definition. From your second link, the definition is:

Perfect forward secrecy (PFS for short) refers to the property of key-exchange protocols (Key Exchange) by which the exposure of long-term keying material, used in the protocol to authenticate and negotiate session keys, does not compromise the secrecy of session keys established before the exposure.

And your third link:

Forward secrecy (FS): a key management scheme ensures forward secrecy if an adversary that corrupts (by a node compromise) a set of keys at some generations j and prior to generation i, where 1 ≤ j < i, is not able to use these keys to compute a usable key at a generation k where k ≥ i.

Neither of these mention networks, only protocols/schemes, which are concepts. Cryptography exists outside networks, and outside computer science (even if that is where it finds the most use).

Funnily enough, these two definitions (which I’ll remind you, come from the links you provided) are directly contradictory. The first describes protecting information “before the exposure” (i.e. past messages), while the second says a compromise at j cannot be used to compromise k, where k is strictly greater than j (i.e. a future message). So much for the hard and fast definition from “professional cryptographers.”

Now, what you’ve described with matrix sounds like it is having a client send old messages to the server, which are then sent to another client. The fact the content is old is irrelevant - the content is sent in new messages, using new sessions, with new keys. This is different from what I described, about a new client downloading old messages (encrypted with the original key) from the server. In any case, both of these scenarios create an attack vector through which an adversary can get all of your old messages, which, whether you believe violates PFS by your chosen definition or not, does defeat its purpose (perhaps you prefer this phrasing to “break” or “breach”).

This seems to align with what you said in your first response, that Signal’s goal is to “limit privacy leaks,” which I agree with. I’m not sure why we’ve gotten so hung up on semantics.

I wasn’t going to address this, but since you brought it up twice, running a forum is not much of a credential. Anyone can start a forum. There are forums for vaxxers and forums for antivaxxers, forums for atheists and forums for believers, forums for vegans and forums for carnivores. Not everyone running these forums is an expert, and necessarily, not all of them are “right.” This isn’t to say you don’t have any knowledge of the subject matter, only that running a forum isn’t proof you do.

If you’d like to reply, you may have the last word.