Your Slack app was uninstalled.

Facts

Works with

Events API
{
    "token": "XXYYZZ",
    "team_id": "TXXXXXXXX",
    "api_app_id": "AXXXXXXXXX",
    "event": {
        "type": "app_uninstalled"
    },
    "type": "event_callback",
    "event_id": "EvXXXXXXXX",
    "event_time": 1234567890
}

This Events API-only event is sent via subscription whenever a Slack app is completely uninstalled.

You should also receive tokens_revoked events for revoked tokens however the order of the app_uninstalled and tokens_revoked events are not guaranteed to be sequential (you could receive a tokens_revoked event after app_uninstalled). An app is not uninstalled until its final user and bot tokens are revoked.

The example above details the complete Events API payload, including the event wrapper. The team_id indicates which workspace uninstalled the Slack app identified by api_app_id.