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. 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.