As a note, Its not 20 percent of people being born. Its 20 percent of people in general, across all currently living generations, are Quirkless. Most of them being of the older gens. In Class 1A's generation, it is extremely rare for someone to be born Quirkless. And you're right, AFO wasn't public knowledge. He also didnt look like a giant Toe 10 years ago. If Inko had the means and the knowledge of "someone who could give Quirks", she likely could have pursued it. If nothing else to save him from the constant bullying and loneliness.