> A team wiki is among the many better options.
Wikis are generally useless unless you have people whose job responsibilities explicitly include creating content for them and keeping them up-to-date. In my experience, if you have your PMs do this then each PM can cover 8 - 12 developers. Whereas if your PMs are just creating and pointing tickets but not also spending a good percentage of their time on longterm documentation, they can cover more like 12 - 16 developers.
Well maintained wikis are fantastic, but you almost never find them because for even a mid-size startup that costs hundreds of thousands or millions of dollars per year. That's the reason for public mailing lists and other email archival products, because you're getting most of the value a well-maintained wiki but with mainly only the nominal cost of paying for the software to keep it running.
I've thought a lot about this (obviously, given FWD:Everyone is in this space), and figured out the actual reason why email is so much better than Slack for knowledge retention; it's because with email you're forced to create the metadata before having the discussion, and then the discussion keeps going until people no longer have anything to say that matches the metadata -- subject, participants, visibility level, etc. (Discussions occasionally go off the rails, but less than 5% of the time as long as the organization provides some basic guidance on expectations for communication.) This metadata is key though because not only does it help you find what you need to do your job, but it helps you figure out what you don't need to read, which is perhaps even more important.
Whereas with Slack you just start talking, and their software tries to figure out what you were talking about after the fact. But despite having an AI team working on this and having invested millions and millions of dollars into the problem, their software doesn't really work at all, most likely because the problem is basically impossible to do a good job of solving from a computer science perspective.
edit: Obligatory self-promotional link: https://www.fwdeveryone.com