Any feedback regarding this pull-request ?

Hey @jacky309,

your PR does not take into consideration the fact that a regular CAN frame shouldn't have a payload larger than 8 Byte. If you want to send CAN-FD frames over the interface, I recommend adding proper CAN-FD support to the interface.
This would entail the following points:

• Adding a flag field to the transmission frame
• sending up to 8 byte if frame.is_fd == False, sending and receiving up to 64 byte if frame.is_fd is True

Do you mean that the serial frame format described at https://python-can.readthedocs.io/en/master/interfaces/serial.html should be changed to introduce a new field to identify if a frame is CanFD or not ? This would be a breaking change which is likely to cause some trouble with existing software, right ? IMO, this is not acceptable.
This PR, as it is now, introduces the improvement that CanFD frames can be transmitted over a serial connection, without requiring any change to the serial frame format. IMO, this benefit outweights the fact that the length check is gone, so I would consider merging this PR for now.