Most use 2.4 GHz now, it’s possible to get into them still if you buy the proper headset for whatever system they use now since they tend to leave it in default configuration. Like my local KFC used a 3M brand system and if you get close enough with one of the headsets it connects to the base and all the others so you can mess with people at the order windows.
Most mainline USA franchises like McDonalds, Taco Bell, KFC, Burger King, Carl Jr/Hardee stopped using the regular RF systems in about 2014. Sonic Drive in was the last one which I could get from almost 2 miles away with the 10 meter quarter wave on the roof of my former house connected to my BCT15x thanks to their base being on the lowband itinerant of 30.84 MHz. By mid 2017 I stopped being able to receive, even took my handheld in to look for upper crossband frequencies of the headsets on the way to home depot for Xmas decor in November of that year. I even went back later again afterhours with the Signal Stalker nearfield mode running and pressed all the order buttons but no hits.
Most worked crossband duplex, with the base on lowband being common and the headsets on VHF-Hi and some on UHF. Not as common but still enough out there were some that split between VHF-hi and UHF instead. There were also some systems that had an intercom frequency of sort, that allowed all the headsets to talk to each other but not the drive-thru so you didn’t have to yell across the kitchen for something. Those were the most fun frequencies because the staff had no idea anyone else could hear them and the sh*t talking between window orders was hilarious, also if someone messed up and sent the wrong item to the front of the kitchen you would hear them catch an earful for it.
I would die to have that back again, I don’t live in a big area so it might be worth checking out independent non-franchised type restaurants that happen drive thrus with an order intercom before the window to see if they have a normal RF system going, unfortunately there’s none in the rundown area I live now (the last one closed in 2013) but everyone else that does live in a city and love business band scanning as well should keep an eye out and give it a try.