What are you disagreeing with?

Even if you do this (and it's good practice!), it is, empirically, not done in the vast majority of codebases.

And even if you succeed with the utmost diligence, a vastly greater number of decisions (those you were not even aware of consciously, or took for granted) will remain undocumented but still be quite real in this "negative space" sense.

Exactly. I couldn't have said it better.