Discord, and 99.99% of companies under capitalism, do not care if they produce more value unless they get to keep that value.
Again, discord improving search would make users rely on their software more, since searching discord for answers becomes a part of user's workflows. Slack offering a premium enterprise "Librarian" feature would probably have significant demand. Usually, companies that offer more value can charge more for that value. I don't see the logic I how gimping search functionality somehow ends up improving these companies' bottom lines.
No. There are websites that copy the content from SO to their Site, and then they SEO themselves to the top on Google, so that the SO Page is lower than the Copy Page.
This is not about the Google blurb.
This could be completely hidden to the Discord devs though. They could very well be slavishly improving various internal metrics. If one of those is time spent in the app, then I could easily see search ending up a lower priority. It doesn't even have to be intentional.
This all likely means fewer paying subscribers.
So, Discord search should be fine for a few months depth, and need not be good further into the past. Exposing historical data to external search engines is even less desirable.
Same of course applies to Slack, HipChat and whatever other commercial chat-like software.