But I also think that if a maintainer asks you to jump before submitting a PR, you politely ask, “how high?”
You might argue that by making rules, even futile ones, you at least establish expectations and take a moral stance. Well, you can make a statement without dressing it up as a rule. But you don't get to be sanctimonious that way I guess.
Total bullshit. It's totally fine to declare intent.
You are already incapable of verifying / enforcing that a contributor is legally permitted to submit a piece of code as their own creation (Signed-off-by), and do so under the project's license. You won't embark on looking for prior art, for the "actual origin" of the code, whatever. You just make them promise, and then take their word for it.