I recently canceled service and switched to AT&T Fiber. It was actually an unexpectedly easy process. But when I dropped off my cable box at a Comcast store, they asked me for my modem. The one that I bought myself. The person taking receipt of my equipment was fine when I said I owned my modem, and just "put a note" on my file.
... then I get a collections letter, explicitly related to the "unreturned" modem. Despite having no history of receiving a rental modem, and no monthly surcharge for modem rental on any of my years-long billing history, they refused to budge. I happened to still have the original box with the serial number on it and that combined with a printout of my Amazon order from back then was enough to at least to successfully dispute the debt on my credit reports. But it wasn't enough for Comcast, and I'm sure I'll still have to pay that bill and put down a giant deposit if (when) I move somewhere that forces me back to Comcast.