It was really weird when Windows tried to start using 'sideloading' as a term when pushing UWP. As if the standard method of installing software on Windows hadn't always been to run an installer file on it.

The term is still in use on the Developer options of Settings where you can decide if UWP apps can be installed outside of the Microsoft Store, though I believe they've backed off drastically, so the default is to 'allow sideloading'.