The thing is that there are times where n+1 isn't a big problem and fixing it would be a form of premature optimisation. I'd prefer to be in control and decide if I care about the n+1 query situation or not for some specific view.
Naturally yes, there's a certain scale where N+1 starts to really matter. And in those situations it might be helpful to be able to enable some feature to warn (or error) on N+1. But optimizing your code to avoid run in fewer queries is often (in cases where prefetch_related doesn't cut it) non-trivial. And presumably when you get to this scale, you're also code-reviewing the jr developers.. so I'm not sure how urgent it is that we all "Ban N+1 in Django" as a rule.
(Or maybe the reverse is better. Ban N+1 by default and the context manager `__enter__` puts back the original assignment, `__exit__` brings back the banned version).