> Can people keep a good mental model of the repo without writing code?

Probably not.

In my experience working on large SW, you can't do interesting stuff without talking to the people who wrote the code that your interesting stuff will touch. And the reason you talk to those people is to get their visceral feel for how their code will be affected.

Ig that’s the case you really need to get some documentation in place. Otherwise after a few older people leave your code can never be changed properly anymore!