Currently, in Python, byte[] maps to a list of builtin.bytes (each list element being of length 1). And I agreed with you it could be just builtin.bytes and not a list.
My understanding is that the proposal from @Zongpei_Jiang is to map uint8[] to builtin.bytes as several ROS 1 messages use uint8 to send binary data around. So this is a different proposal I think, and I’m not sure that this should map to builtin.bytes in that case.
that’s been on our list for a long time but we didn’t get around doing it yet.