I haven't used Zulip but based upon the public documents and selling points. Seems like it's great for organizing within a corporation. But for an open source project, I feel the "pain points" they talk about are non-existent. Although I do agree that much of the conversation/knowledge is lost in these types of chat applications. In the past, if I had a problem with an open source lib. I could easy just search for it and it might link to an open issue or a forum post. Now for each problem, I might be lucky to see a similar issue but for the most part is now resolved by going directly to the project's preferred chat platform and asking the question there.