As in it's not a scam, it's also common for products past as little as 2 years end up with no support (nod to "Smart TVs"). And while I'd love to be disproved, another HP tech will likely repeat your first experience.
If Costco doesn't take them back, I consider Ebay to be the next best thing.
WHATEVER YOU DO, DO NOT FALL FOR FAKE OR SCAM SUPPORT SITES.
https://www.google.com/search?client=opera&q=OfficeJet+8600+"SOLVED"+Regional+Reset finds others were able to do a regional reset but your issue sounds different as in you want to use cartridges that are not for this model or such.
I had the same issues that others have had. I have been purchasing my cartridges and printer from Costco, and somehow installed a batch of the Z20 cartridges in my 8600... The colored cartridges finally ran out, and I purchased the new 950XL and 951XL cartridges, and these are Z10. I read all the notes about the regional reset, and reached out to tech support. They tried to reset, but the tech support person said that it failed, and gave me the option of purchasing a new printer from them. (I think there was another post that mentioned the same thing.) The discount they offered me was the same price they were offering the printer for online - so no additional discount. They would replace the ink cartridges that I purchased with ones that were compatible with the new printer... and possibly extend the warranty an extra 3 months.
They were not able to tell me when the 8600 was discontinued and how long they supported discontinued product. They sounded like they just wanted to sell a new printer.... pretty bogus tactics imho.
Anyone have any suggestions - if I call back, do you think another support person might be able to get the reset to work?
I'm using a mac, and in another posting, someone said that doing the reset from a mac was not possible. Does HP have people calling in to reset the region, just a tactic to sell you a new printer?
I'm not sure if printers are considered an electronic device, but hopefully Costco will take this boat anchor back along with the ink cartridges. If anyone has recommendations for a good color laser printer, maybe it's time to upgrade from these inkjet printers.