Have there been any blind tastings for that? I mean, if people swear by beef tallow's taste, they should be able to also prove it experimentally.

This would also set a level field between beef tallow and other oils. I would expect that a lot have changed in the fast food industry supply chain since the "good old days". Frying oil is only one of the factors that may have affected the taste of fries. Not to mention that everything tastes better when one is young.