I am intrigued by the idea of product-specific wikis, as outlined in North Star Product Management. The idea is to have a living document that explains how each product component works, why it exists, why certain decisions were made, and what the future looks like:

The purpose of North Star Docs is not (at least, not necessarily) to minimize changes — changing the North Star Docs is just fine, and is of course actually encouraged, if we can find a better way to do something.

The purpose is, instead, to help map complexity for systems-level, rather than localized, problem solving. They are comprehensive and detailed specs like a Waterfall, yes, but they serve a very different purpose.

This can then be paired with project-specific specs as and when needed. But separating those from the main “this is the overall vision of the product/component” document provides a clear decision-making framework and builds organizational memory.