By default, KWin does not have shortcuts for the Increase/Decrease Opacity of windows but it does come in handy often for some people, such as myself so I suggest they be set to the following.
Increase Opacity=Meta+Alt+=
Decrease Opacity=Meta+Alt+-
This is similar to how the Zoom function works with Plasma but adds the extra Alt modifier.
This is the setting I use for my workflow and I use it often when podcasting so others might find it useful and it doesn't negatively affect anything as those shortcuts I'm suggesting are not utilized for anything at the moment.