I worked at a startup where it was a running gag that we had an "all hands" every two weeks and somebody put their hands up and said that they couldn't find documents in the places we kept them and suggested the answer was adding a new place to store files -- somehow we never got insight into a group about this.
There are two sensible answers to this problem:
(1) Treat it as a wetware problem, that is, if people are well organized in a team they are going to figure out what their business process is and stick with it regardless of what tools you use.
(2) Treat it as a technology problem. The obvious thing here is some kind of system which can not only search over a large number of "documents" (e.g. a message on Slack is a tiny document) but understand the relationships between the documents.
There are two sensible answers to this problem:
(1) Treat it as a wetware problem, that is, if people are well organized in a team they are going to figure out what their business process is and stick with it regardless of what tools you use.
(2) Treat it as a technology problem. The obvious thing here is some kind of system which can not only search over a large number of "documents" (e.g. a message on Slack is a tiny document) but understand the relationships between the documents.