Function restorable 

async fn restorable(
    services: &Services,
    deactivated: Option<&Arc<Map>>,
    user_id: &UserId,
) -> Result<bool>

Reports whether the account reads as deactivated here without the foreign column marking it so.

The empty password a foreign database gives an account authenticated elsewhere is the byte pattern this server writes for a deactivation, so the foreign marker is the only thing separating them. A row neither side can read is reported rather than guessed at.