Both in Kubuntu 19.10 with plasma 5.17.90 and Kubuntu 20.04:
Adding widgets:
I had trouble adding the icon only task manager to my plasma panel. The widget was added but was invisible! So I kept adding the widget only to later realize I had several of them! I had the same experience with the plasma transparency button widget.
Removing widgets:
This video by "someone else" from 2015 is still relevant. Getting to the "remove" bit is still hit-n-miss for me.
Adding widgets:
I had trouble adding the icon only task manager to my plasma panel. The widget was added but was invisible! So I kept adding the widget only to later realize I had several of them! I had the same experience with the plasma transparency button widget.
Removing widgets:
This video by "someone else" from 2015 is still relevant. Getting to the "remove" bit is still hit-n-miss for me.