The Electronic Frontier Foundation has posted a detailed
study on third-party corporate surveillance on the Internet (and
beyond). “Both Google and Apple encourage developers to use ad IDs
for behavioral profiling in lieu of other identifiers like IMEI or phone
number. Ostensibly, this gives users more control over how they are
tracked, since users can reset their identifiers by hand if they
choose. However, in practice, even if a user goes to the trouble to reset
their ad ID, it’s very easy for trackers to identify them across resets by
using other identifiers, like IP address or in-app storage. Android’s
developer policy instructs trackers not to engage in such behavior, but the
platform has no technical safeguards to stop it. In February 2019, a study
found that over 18,000 apps on the Play store were violating Google’s
policy.“

Source: LWN.net – Behind the One-Way Mirror (EFF)