>I want sideloading for my own purposes... On the other hand, I absolutely don't want my older relatives to be able to install stuff from wherever.

The Mac preference pane included in the article implements this. My parents aren't going to change that setting unless I tell them to which I won't.

And don't be ageist: my pre teen kid wouldn't either, having already rendered his computer unusable a couple of times in other ways. Once he became a bit older things were different, but by then it didn't matter to me and wasn't really any of my business.

It should be added to iOS and tvOS.