I ask because I know now for a fact that AT&T doesn't put "financed" phones on a "blacklist." For example, if someone buys an AT&T phone on an installment plan and then decides to walk away from the contract before the phone is fully paid off, that phone can still be used on the AT&T network, including on MVNOs. You can even use the phone as a trade-in if the phone still has value. The only thing that AT&T will do is deny unlock requests, so you can never use an AT&T financed phone on another network.
I'm fairly certain that the opposite was true of Sprint, before T-Mobile bought them. Years ago, I wasn't able to activate a Sprint-locked phone on Tello due to the phone being financed. Fortunately, I was able to return the phone under eBay's seller protection policies.