Google bans Android apps that show ads on your lockscreen, there's a catch however
Begone, lockscreen ads! - Google, late 2017
In a rather user-friendly move today, Google has changed its Play Store policies and no longer allows apps that place obtrusive ads on your handset's lock screen. The new change affects all apps whose purpose is any other from being lockscreens themselves and is detailed in the the developers' monetization support page. It reads as follows:
"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."
This means that if an app is designed to be a lockscreen replacement, it's technically still allowed to show you ads; however, music players, third-party galleries, and casual games won't be allowed to do that.
Now what remains to be seen is Google actually becoming the bad cop and following through on its new guidelines by pulling out apps that are not compliant with them.
source: Google
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Things that are NOT allowed: