There are a multitude of rules concerning backstory in novels. How much to include, when to insert it, why it needs - or doesn't need - to be there, and of course what not to do with backstory. You know how I feel about rules, but let me attempt to be reasonable about this element of writing stories.
First of all, rules are guidelines for those writers who appreciate them, who actually need them, and who want to pass all the "tests" which will lead them to a publishing contract while hoping to improve their skills. Rules are borne from the theory that setting up general requirements to write well will in fact produce better writers and stories. Most professionals will admit - when pressed - that rule-breaking can be done effectively by good writers without those rules being missed.
And that's the defining factor. Good writers can do anything and make it interesting. They can shove backstory in where it supposedly doesn't belong and the reader doesn't even notice. They can fill page after page with backstory, but they manage to set it up so well and keep it uniquely enticing so that the reader is carried back into the past of a character or circumstance without objection.
Not everyone can do it. Thus the rules.
Backstory doesn't bother me. Neither do prologues. Or adverbs. Or a multitude of other items considered to be no-nos. Just do them well. That's the quantifying and not-so-simple solution we writers strive to do.
Father, thank you for words, even though they hide from us at times, feel completely inadequate for our true expressions at others. You are the supplier and provider of all we need. Thank you for your generosity to us and for imparting inspiration for what you have for us to do. In the Name of Jesus, Amen.