It doesn't make any sense for large group chats: there's just no secrecy to them. And that will cut you off from desired capabilities like server-side search. Not that I think that Zulip makes any sense in general when there are federated protocols that work and scale as well, but that's another discussion.

I don't see how E2EE would be that important for Zulip. It is mostly used in Teams who work together on a project or for a specific business/company. They all know each other (or at least part of the Teams) and share the information. And it is (typically) run on dedicated hardware owned by the Team.

So i personally see no benefit for E2EE in these kind of contexts.