Locking knowledge behind something that isn’t publicly searchable or archivable works fine in the short term but what happens when Discord/Slack/whatever gears up for an IPO and limits all chat history to 1 week unless you pay up (oh and now you have a bunch of valuable knowledge stored up their with no migration tool so your only options are “pay up” or lose the knowledge).
At least people treat IRC as ephemeral and place all documentation elsewhere. People are writing whole wikis inside of Discord that are not publicly searchable.
Local logging doesn't help much for searchability when you're new and it requires you to be online 24/7. Anyway, that's beside the point. Even if IRC had built-in server history it still has the same problems but I never saw people being outraged about it.
There's a whole FOSS ecosystem of chat/collaboration applications, like Mattermost and Zulip; there's Matrix for a federated solution, and tried-and-true options like IRC.
For something called "Reclaim the Stack" to lock discussion into someone else's proprietary walled garden is quite ironic.