It's worth noting that the cost of storage wasn't the issue in this case. They already had a system that allowed you to determine which headers in the Vary list

It's worth noting that the cost of storage wasn't the issue in this case. They already had a system that allowed you to determine which headers in the Vary list would be respected and so you could calculate a worst-case storage load. I mean, it definitely was an issue in general and we were careful about avoiding the same content being stored multiple times but it wasn't the reasoning they communicated behind the change in the anecdote I related. I think the best suggestion was in another thread by @johncolanduoni where he pointed out the difficulty of storing, distributing and retrieving the metadata per-resource that would be necessary for each PoP to correctly determine the Vary requirements at request time.