So far, searching email has been more effective than searching anything else (Wiki, Slack, etc).
IME, the majority of long emails should not be persisted in such places, because they contain a fair amount of errors. One could say that any long email thread should be eventually synthesized in the KB/wiki after the conversation has wound down.
And Slack/Teams? No way. If I need to write something long in response to a Slack IM, and email is verboten, then I'll insist on setting up a meeting.