At my place of work, our internal chat rooms (not Slack, but a similar product) are automatically wiped of messages older than two weeks, to actively discourage anyone from using it as a knowledge store.

Instead, we have a company-wide BookStack instance that's the primary store of documentation, split into different sections for project information (per project) and rough notes (per employee). Everyone is expected to contribute to the former, and may also add to the latter if they wish to share something that may be useful to others.

Usually, helping someone else out with the information they need involves pointing them to the right page where it's already documented, or writing something up for them. Perhaps also talking them through it one-on-one if it's complex.

This works for us because we're required to provide reports to our customers every few weeks, in regards to what we've been working on in the each contract - in addition to any actual deliverables. Keeping everything well documented internally makes it much easier to document for our customers.

This is a small company of around 40 employees. Not sure how well this will scale as we grow, but it's been working well so far.