Companies that cater to app makers have found ways to game both iOS and Android, enabling them to figure out which users have uninstalled a given piece of software lately—and making it easy to pelt the departed with ads aimed at winning them back.

Adjust, AppsFlyer, MoEngage, Localytics, and CleverTap are among the companies that offer uninstall trackers, usually as part of a broader set of developer tools. Their customers include T-Mobile US, Spotify Technology, and Yelp. (And Bloomberg Businessweek parent Bloomberg LP, which uses Localytics.) Critics say they're a fresh reason to reassess online privacy rights and limit what companies can do with user data. "Most tech companies are not giving people nuanced privacy choices, if they give them choices at all," says Jeremy Gillula, tech policy director at the Electronic Frontier Foundation, a privacy advocate.