New: Perhaps the most comprehensive report on how mobile apps secretly share location data with companies most people never heard of, who then sell it to all kinds of interested parties from advertisers to US military contractors, by @seanodiggity+ @U039b:
https://www.expressvpn.com/digital-security-lab/investigation-xoth

Details on the findings on GitHub:
https://github.com/expressvpn/xoth_location_tracker_investigation

Articles:
https://techcrunch.com/2021/01/28/x-mode-location-google-apple-ban/
https://www.vice.com/en/article/epdkze/muslim-apps-location-data-military-xmode

In December, Google+Apple announced to ban apps that embed tracking software operated by X-Mode, a location data broker who has sold location data to US military contractors.

The investigation found 199 apps with X-Mode embedded, 1 billion downloads, 90% of them still available.

Many more apps than we previously knew of contain tracking software by X-Mode, including muslim prayer apps, dating apps, navigation apps, travel guides, weather apps, compass apps, games, fitness apps, QR code scanners…

Here's the full list of 199 apps:
https://github.com/expressvpn/xoth_location_tracker_investigation/blob/main/xmode.csv