Answer
Can a website know if a user uninstalled its PWA?
There is no standard browser event that fires when a user uninstalls a PWA, so a site cannot reliably detect uninstalls directly. In practice, teams infer it indirectly — for example, a push notification send failing with an "unsubscribed" or "gone" error from the push service, or a previously frequent user simply stopping all visits, are the closest available signals.
What is not possible
There is no JavaScript event equivalent to a native app’s uninstall callback; the page has no way to run code after the icon is removed.
Indirect signals
A push subscription becoming invalid (the push service returns a 404/410 on send) is one practical, if imperfect, signal that the PWA — and its subscription — may have been removed.
Related event that does exist
The appinstalled event does fire when a user installs the PWA, giving reliable install analytics even though the reverse uninstall event does not exist.