I saw some pepsi ball locks being used for their intended purpose at an outdoor event in Palmyra, NY back in July. I have called and talked with Pepsi reps about renting for an event and they do still rent the equipment to anyone, in fact they were running a promotion at the time that if I were to purchase 3 kegs of soda ($25 each), then the fountain dispenser would be a free rental. I didn't get to keep the kegs, though. I asked if they would sell me empty kegs and he said he would have to talk to his manager, I got no response from him on that. Pepsi reps in other places have told me that they still distribute in them for events and concessions trailers and such, but they don't sell empties. The reason is that Post-Mix soda setups require a water line to run, Pre-Mix setups do not. If someone is running as soda fountain where there's no access to running water, they'll need a pre-mix setup.
<personal speculation>I think the general idea is that they don't see as much usage as they once did. There was a point where most of the bottlers liquidated the excess, but still kept a limited number for limited circulation. Occasionally, as some of their supply wears out, they'll swap out for new, allowing a trickle of used kegs back out into the used market. Nowhere near the flood that occured in the past few decades, but enough that they won't be extinct. IF concessions trailers that have no water access somehow find a way to switch to Post-Mix Bag in Box, then there will be no need at all for Pre-Mix cornies, and at that point, they will slowly die out completely. But so far, there are some situations that still require pre-mix setups, and will still require cornies</personal speculation>