You seem to be under the impression that this filter is based on message content. It's not. All content is e2ee, and this filter is not downloaded to the client

You seem to be under the impression that this filter is based on message content. It's not. All content is e2ee, and this filter is not downloaded to the client. The filter is to stop abusive network behavior, e.g., someone enumerating every phone number to get a list of all Signal users. If you'd like, I can link you to some of the research that has been done to determine some of what they're doing (or you can search google scholar). edit to add: I guess I should clarify, the messages in Signal are properly e2ee with ephemeral keys, and there is no content hashing or the like to leak anything to the server. It's not public yet, but if you'd like, I can share some of my research into the structure of an actual Signal message, from the TCP layer down to the encrypted payload.