In any case, this was in the context of app stores. What I meant is the ability to go flip a switch that enables sideloading, as you can do with Android or Win10 Store apps, if you know that one exists - on the assumption that if you do, you also know the risks. On Win10 specifically, this is stashed away in the system settings page titled "For developers", which comes with a bunch of warnings, so it's not something you can turn on by accident.