This is an applet for Plasma 6 that allows you to toggle the transparency of panels. This was ported from the original Plasma 5 applet by psifidotos, who appears to be inactive. You can find the original applet this is based on here: https://store.kde.org/p/1338479

To use, add the button widget to your panel and activate it. Your panel will become completely transparent; just exit the panel configuration mode to hide the button.

You can also set a keyboard shortcut to toggle transparency at any time. Just right click the button, select "Configure", and specify the shortcut you want to use.