I don't think you're being stupid, no. I agree that it's an informal term... I don't know of anyone who would disagree, but maybe that has something to do with where I live. I use y'all and "you guys" (or yinz when I want to be obnoxious) when I want to pluralize "you," but I never use it in a formal context, as a rule.

But I do kind of wish that English had some form of plural "you," like other languages. I feel like we're missing out.

"And death is at your doorstep
And it will steal your innocence
But it will not steal your substance
But you are not alone in this"

“My ability to turn good news into anxiety is rivaled only by my ability to turn anxiety into chin acne.” - Tina Fey