Google is letting some apps continue to display ads, however. "Unless the exclusive purpose of the app is that of a lock screen, apps may not introduce ads or features that monetize the locked display of a device," the new developer policy note reads. In other words, if the express purpose of the app is to be a lock screen, it can continue to annoy you with ads.
Last year, users on Reddit went so far as to build a spreadsheet of apps that exhibit the behavior, which includes the Go suite of apps (Weather, Keyboard, etc.), ES File Explorer, HiFont and 360security. On a Reddit post about the new policy (with over 1,400 comments), users noted that even smartphone manufacturers like Samsung have pre-installed apps that exhibited the behavior.
Bad ads have become a scourge across Google products, and the search giant said it removed over two billion of them last year. Apps that are deceptive, disruptive, inappropriate or interfere with apps or device functionality and banned from the store. Google introduce Play Protect early this year in an effort to curb the problem, but a lot of shady stuff still gets through.