[heinrich5991]

Some random comments: I'd say this is something that wouldn't have happened for Signal. The comment I linked didn't make it sound accidental. In the linked issue thread, they talk about aggregation done by the server, which means that the server would still be able to tell that person A, B and C reacted with the same emoji. That sounds like a lot of information leakage to me, e.g. for people who do votes via reactions.

[feanaro]

Signal's and Matrix's position is quite different because Signal doesn't attempt to be a distributed eventually-consistent data store but simply a message transport. This is a trade-off which costs you some metadata leakage and is what leads to aggregations being a thing that is relevant for Matrix. It also gives you a lot of power, because you can now construct generic, distributed E2EE-enabled apps for "free".

That being said, there is still a lot of it that is up in the air. From what I've gathered, there's been talk about leaving aggregations to be done client-side specifically for reactions.