This is according to a post by Google in which they have announced that come 7th of January, 2019, they will be ending the SMS notification feature for Google Calendar. Instead they are asking users to simply rely on in-app notifications for their reminders, although they don’t really mention why they are ending this feature.
According to Google, “Starting on January 7, 2019, SMS notifications from Google Calendar will no longer be available. Since Calendar offers in-app notifications, you can still get notified, regardless of your device or connection. If you had previously been receiving Calendar notifications via SMS, they’ll be replaced with in-app and email notifications.”
For those who might have disabled calendar notifications for Google Calendar, it seems that now might be a good time to turn them on, lest you forget and come 7th of January, 2019, you start missing out on event reminders because the SMS feature has been removed.