It's starting to look like the backlash is making them dial it back. Initially it was believed it would require an adb command to unlock it, but it's starting to look like it will just be a warning, not that far off the current status quo. One hopes they come to their senses.

Android Authority

Here's how Google's getting ready for Android's upcoming sideloading restrictions (Updated)

Ahead of Google's mandatory developer registration for sideloaded apps, the company's getting the Play Store ready.