Technically illegal where I live.
In Brazil you can't sell a device with a given feature and then remove said feature in a software update. Even Apple, known for never allowing downgrades, was forced to downgrade and pay a fine to a customer after his iPad 3 updated to iOS 7 and lost an iOS 6 feature.
In other words... every single Android device sold until today in Brazil allows sideloading. Even if a single customer uses a sideloaded app, removing the ability to sideload freely would be illegal, and because the original feature didn't require a developer signature it can't be enforced now.
The issue is, as always, if this went to court somebody would have to manage to explain to a tech illiterate judge what a "developer signature" is, how this relates to "sideloading" and so on.