An undocumented codebase has always been a tax on new hires. In the agent era it's a different problem.
Agents don't go quiet when they hit a gap in context. They fill it. Confidently, and wrong. The architectural decision everyone knows because they were in the room? That doesn't exist to the fleet. They'll reverse it, or build on top of it in ways that silently invalidate it.
Write down why the system is the way it is. Not for the humans. For the context window.