Adds a button to the toolbar to toggle whether new layers are created with alpha inheritance enabled. The option can also be toggled in the Layer menu, or bound to a hotkey under Scripts > New Layer Alpha Inheritance.
Download newlayeralphainherit.zip and install by going to Tools > Scripts > Import Python Plugin From File... in Krita and selecting the zip file.
If you're having issues, more information on plugin installation can be found here.