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.