Camelbacks/hydration packs

Can anyone confirm if Camelbacks (or similar hydration packs) are allowed in the festival grounds (not campgrounds, I know that’s fine). The FAQs aren’t helpful and I’ve seen conflicting things here/online elsewhere and even heard conflicting things from friends. A friend who has gone every year for the past few years (except last year) says it’s fine but another who went last year said he couldn’t bring it in?? What’s the deal?!