We use an internal forum in addition to chat. Anything substantive goes in the forum. We'll regularly reference 5 year old posts.
This article is really, "We couldn't adapt our processes to Slack, so we blamed the tool and replaced a chat app with processes, because we're special snowflakes and how could anything Not Invented Here apply to our totally unique problems of communication?"
Sure, they ended up with a series of other tools to cover their workflows, the thing they "invented" was how those tools work together to cover the gaps created by not actually having a comprehensive and well-configured chat app.
IMO just use forum software if you're going that way. Much more low volume, too (generally).
Further, "optimize for searching" isn't a thing. Teams gives full text search, and you can find any word or phrase you need to instantly.
I think you're optimizing for familiarity.
For example https://chat.zulip.org/#narrow/stream/3-backend/topic/Vagran...
I'm curious why people don't like forums for discussions.
Proper threading.
Read/Unread tracking.
Keyboard UI. (And the mouse UI tends to be bad as well.)
Responsiveness. (Especially: page loads which are not instantaneous.)
Killfiles.
They generally have mediocre search functions unless they are entirely visible to Google.
These are off the top of my head.
http://home.httrack.net/~nocem/
Thunderbird does this by default. Reddit breaks threading when sorting by "new". On HN, you have to search the comments page "minutes ago" or "hours ago" to find new comments in an existing thread.
Instead, we went for XenForo, made by the original vB developers. It's got just enough modern technology to make it a bit more pleasant, and it's super fast (faster than vB 3). Migrating vB to that was easy enough too (took a while, but faster than Discourse which needed three days for the basic import and then still had a million tasks to run in the background)
We really wanted to to like Dicourse, but everyone on my team found some little thing that really annoyed them ("why does it hijack ctrl-f?", etc.). It was death by a thousand cuts. XenForo is the most modern of the traditional forum platforms that I reviewed. Everyone just gets it.
I'm convinced that tight integration between source control, issue trackers, wiki pages, and forums is a good thing. Generally you can use URLs, but that's often cumbersome.
It used to have a halfway decent forum addon, and the ability to have the wiki features/content embedded was enough to convince us not to try and use a separate forum.