Earlier quoted context omitted.
This is a depressing trend in itself, and not just for QA setups either. Way too many communities seem to exclusively post on login only walled gardens like Slack and Discord, where stuff gets lost very easily and search engines can't index anything. As you said, it makes it very annoying to find said information later, especially if you're not already part of the community/don't know where to look.
I heard someone a while back mention that he treated discord like verbal discourse: ephemeral. Perhaps not everything needs to be logged.
But a lot of communities use it for a lot more than basic chit chat. I've seen open source projects where relevant info is only shared on their Slack server and nowhere else. Companies often get useful resources shared on their Slack instances that never gets backed up elsewhere for later.
And in the gaming world, it seems like Discord is the default method of communication for the game development, modding, speedrunning and hacking/datamining communities. For instance, most of the relevant techniques used for speedrunning Zelda Breath of the Wild are only found in the Discord server, or a Google Docs linked from it.
There needs to be a separation of the ephemeral from the non ephemeral here, and communities need to share the latter on publically accessible sites instead.