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. The same in Mint 20 (also Xfce 4.14).
In previous versions (e.g. in Xfce 4.12 from Mint Xfce 19.2) that used to be done through an "alpha option" slider:
That slider is absent in later Xfce versions like 4.14:
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?