In my experience Slack just doesn’t work as well as Discord does, generally speaking. It’s good enough to put up with for work chat, but the bar for the general public is higher.
How? I look through my discord search settings and it’s pretty straightforward. I search for something a friend posted some months back and found it pretty easy. The only difficult part is knowing which server it’s on.
Discord uses default elasticsearch settings for search, so almost every word belongs to an equivalence class of synonyms and conjugations that are often not equivalent or mutually relevant in technical usage (for gaming-related use or for work-related use!), multi-word phrase search is impossible, searches including words in the stopword list are impossible, searches including programming language syntax are impossible, etc. "ide" is a fish or an Integrated Development Environment but belongs to the same equivalence class as "i'd" (contraction of "I had" or "I would", mysteriously NOT on the stopword list!) or "id" (part of the mind in Freudian psychoanalytic theory). If you go to a server concerned with programming, psychoanalysis, or fishing, and you search for "ide" or "id" you probably weren't looking for every single time anyone ever said "i'd."
So you use something designed for gaming to run a technical community, and when the search wasn’t built to handle highly complex or technical queries you complain?