Apple Support is unable to assist with this. Only the company supervising the device can remove it.
That said, a supervised device and a managed device are not the same. Some supervised devices are managed, and some managed devices are supervised. Best practice is that all managed devices are supervised.
If you look in Settings > General > VPN & Device Management, if nothing is there, the device isn't managed. That said, if the device is not released properly by an organization, wiping the device could return it to a managed state.
The OP should attempt to wipe the device and set it up as a new device. This will show whether the device is still connected to management or not. If released properly, it will remove the digital watermark that says the device is supervised by the org.