I have seen no indication of system design documents falling out fashion. If anything, the problem is the initial, pure document is used as a reference for too long instead of proper up-to-date documentation.

I'm currently putting together a design document (very simple system and still high-level) to ensure my idea is communicated concisely and unambiguously.

The main motivation is I'll have a refined reference to point people to so I don't have to worry about repeating context to all possible stakeholders.