@jerger @steve
Keeping msgs immutable is a general best-practice, I gather.
In the case you mention it becomes confusing to still use client/server terminology. You have a full actor on the client's side, and when it sends a msg it acts in server/S2S role.
Btw, in that scenario we do not have to make the distinction client + server anymore, as we have just actors communicating with each other. Then we can think in terms of the actor model, and honor its qualities.
A client sending to the server's outbox is then analogous to an actor sending to another actor's inbox. That is a one-way msg exchange usually, fire and forget (esp. in a pure event-driven architecture... which the current fediverse is not). The remote actor is not responsible for keeping the Activity (event) in its server-outbox / actor.inbox. That corresponds to the spec part "may disappear at any moment".