You've always been able to use any of the web browsers in the store to download and install a random APK from a website (for example F-Droid), you don't even need to sideload it. Sideloading apps is mostly just a relevant concept for developers or for users who have no alternative to getting custom code on a device. (Edit: Speaking of ad fraud brought up by the GGP, there are also many automation apps, at least one (Automate) uses a plugin flow-chart architecture exposing all sorts of functionality, with users able to share custom scripts. Not to mention tons of plain "auto-clicker" apps.)