A few thoughts in random order.
Most search engines will have stackoverflow and/or discourse in the top five hits or so. For FAQs, you will find a popular and well formulated answer on stackoverflow. For more “localized” questions (as they say), you will find a stackoverflow answer with a couple of votes, sometimes of high quality (in my experience, DataFrames questions have great, updated answers, to give one example) and sometimes (too often?) pertaining to an old version of Julia and out of date. I have personally attempted to get these updated every time I have come across them, but with my limited breadth of incomplete knowledge it has not always been possible. The good thing though is that the (small number of) people involved in these have an incentive to update their answers. “Pinging” them with a comment is sometimes all it takes (to clarify: if you leave a message in the comment section, the author receives a notification, which they often notice and react to).
Zulip is a completely different environment, with very dynamic, short chats, where you can quickly get advice from some advanced users who selflessly share their time and knowledge. These are typically super “localized” and most of the time not for general consumption. Zulip’s purpose is clear and it works great for its purpose.
While my experience is limited, I see discourse as a halfway house. At times it functions like a chat with random off-topic bifurcations and a high rate of depreciation. At other times it functions like stackoverflow with a “like”/reward system and a “solution” selection system. It works great for recent questions, where the solution is up to date. Unfortunately once an answer becomes out of date, the general practice is to discourage reviving and updating the answers. Unfortunately search engines keep picking them up, with the result that you waste time reading outdated exchanges.
I think all these platforms can coexist, but improvements could be made to how search engines “pick up” answers posted on stackoverflow and discourse. I think it would be great if more Julia users visited stackoverflow, made edits to answers, and upvoted them to make them more visible to search engines. I also think the problem of stale answers on discourse must be addressed: One solution would be to “close” or “retire” obsolete threads by dumping them in an “archive” where search engines would not easily find them.