Wait really? I've never seen that happen on Discord, I can still see messages from over half a decade ago. Why would they delete data they mine?
Slack on the other hand deletes everything immediately if you're not on the paid plan of course.
Main point was that you can't access or view it without creating the account at first, or even being able to find the whole server. You might even need to give phone number before you can view anything. Then wait 30min to verify that you are not a bot. And all this to find a solution for a problem which you might never return to. The search in Discord is kinda terrible.
Also, if you are couple days or even hours late for the discussion, it might be too late to take part or continue it in large communities. Discord lives in the moment, it is not a place to store information.
On the other hand, on forums, it is not a problem to continue some discussion couple days later. The quality is often much much higher.
But yes chat is chat, you can't compare it to a forum style board because they're not trying to do the same thing. You do typically need both a Discourse and a chat server for any major organization.