When will the service no longer be operable for users?

Partial service outages do not count.

Full unexpected outages do not count unless it is expected by multiple credible sources that the service will not be fixed.

An intentional shut down definitely counts.

Constitution is not identity, as in any modifications to the service such as features or name changes (X) does not count as Twitter shutting down. However if a new service is launched to replace the existing service it will be considered a shut down if there is continuity only in name, user base, or user data.