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iFixit Tears Down McDonald's McFlurry Machine, Petitions Government for Right to Hack Them
(www.404media.co)
This is a most excellent place for technology news and articles.
How do they get the data?
They reverse engineered the ordering API (that the app uses) and try to add a McSundae to an online order. If the ice cream machine is broken, it won't let you add that item to an order for the specific location. If the McSundae machine is working, it will let you add that item to the order for the designated location.
It updates each location every 30 minutes, so is very up to date.
More details about it here: https://hypebeast.com/2020/10/mcbroken-site-mcdonalds-ice-cream-machines-working-tracker-info
If its the one I know, it sends an online ice cream request through the API, and then cancels the successful ones before they reach the stores. The unsuccessful ones are broken machines, essentially.
iirc it used to get it through a reverse engineering of the McDonald's app api. Not sure about now tho