Advertisements is the life of some apps but some Ad implementation are very annoying and Google has totally disallowed them from the playstore.
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
The policy is more confusing as “No Ads on the lock screen” was stated.
So, any Apps that bills itself as a VPN, Photo editor, File sharing and others are not allowed to have a lock screen Ad but apps that the main function is to monetize with lock screen e.g Sliide Airtime, are allowed to use lock screen. Presumably, you know what you’re getting when you install a lock screen app.
Google will follow through and start pulling offending apps from playstore.