Even before the current rules partitioning cache by domain, has the promise of those cached scripts ever paid off? Even when every website would use just jQuery, everyone had a different version, getting it from a different CDN, etc...
Look, sometimes we need to tell ourselves stories to make ourselves feel better about the amount of Javascript we're making people download to access our blogs.
Probably not. A few requests surely got cached, but the vast majority likely did not. jQuery has its own official CDN URL so that probably had a chance at being deduplicated.