It's been assumed that questions about resources are off-topic because they aren't questions about the language per se.
Here's an example:
The following Meta.ELU question looks like a duplicate (of this Meta question), but it's not:
That question asks whether questions about resources are on-topic, but I'm interested to know where a visitor would look to know that 'resource' questions are off-topic (if indeed they are). The accepted answer there recommends asking at Writers.SE, but the attempt to do so was apparently rebuffed.
The following Meta.ELU question also looks promising:
It was answered by a current mod (although he wasn't one at the time the answer was proffered), and offers the opinion that 'resource' questions are on topic. He prefers that they be asked on Main, but concedes that by the time he joined EL&U, the decision had already been made that 'resource' questions should be asked on Meta rather than Main. However, that answer doesn't indicate what standard documents (FAQ, for example) a visitor could browse to know that 'resource' questions are off-topic on Main.
Where in the documents accessible from the question-mark menu does it indicate that questions requesting resources are off-topic on Main.ELU or Meta.ELU?
It's no good pointing to a consensus on Meta.ELU because that would be like looking for a needle in a needle stack for new visitors. A consensus answer on Meta.SE would be even worse - the visitor might not even be aware of that particular needle stack.