https://arstechnica.com/gadgets/2017/12/google-bans-android-developers-from-adding-lock-screen-ads/

Google is cracking down on one of the most annoying ad types on Android. As first spotted by Android Police, the Google Play Developer monetization rules have been updated to ban lock screen ads. Here's the new section:

Unless the exclusive purpose of the app is that of a lockscreen, apps may not introduce ads or features that monetize the locked display of a device.