My experience is this: Linux Mint Xfce 19.3 came with a Xfce panel version (4.14.1) in which transparency cannot be set anymore.
That used to be done through an "alpha option" slider:
That slider is absent now:
This would be normal if compositing were disabled, but it's not (can change windows opacity). It could be a new feature or a new bug.
Is there a way of getting a transparent Xfce panel in this case?