Discord is also an especially bad choice, at least for two reasons: it is not searchable, and everything might be gone at any point for any reason, when discord or its automated algorithms feel like it.
As someone who participates in relatively high information density Discord servers (mostly ML related), search is rarely a problem for me. Yes, you need to do keyword search, but it's not the end of the world-- whenever I've needed to find a message 1-3 years back, I've found it successfully. Even in servers that I'm relatively new to.
I believe they meant "not searchable from an external search engine", which is a serious issue in many people's view (including mine). I would also suggest that it's a problem that chats are not browsable, because if you're new to a particular problem, you may not know what keywords you need to search for. With a forum, there are categories you can browse and usually discover the keywords you need much easier.
The assumption is that forum is viewable by guests (including Google) and Slack is not, so to search the Slack history you need to log in to Slack and use the Slack search (which might be broken / paid in the future, etc)
