According to an updated support document regarding the SMS calendar notification service, Google will be discontinuing that service on June 27th, 2015. SMS notifications were the original method of getting SMS alerts from a Gmail calendar prior to the smartphone explosion and still the only way for the remaining dumb phone or users to get active calendar notifications on the go.
Its unclear how many users are still taking advantage of the free service, which even now still has instructions on how to setup immediately following the notice of shutdown, but it is a service that Google has offered for free and likely has a very real cost attached to it.
It is noted that this change does not affect Google's business or enterprise customers, both paid and free, and that this move likely has more to do with pushing users towards the smartphone platforms that Google has been so successful at monetizing.
Important: SMS notifications not available after June 27th
Starting on June 27th, 2015, SMS notifications from Google Calendar will no longer be sent. SMS notifications launched before smartphones were available. Now, in a world with smartphones and notifications, you can get richer, more reliable experiences on your mobile device, even offline.
To receive notifications on your smartphone, you can use the calendar app that came with your device, or install Google Calendar for Android or iPhone. Learn more about notifications.
Note: This change will not affect Google Apps for Work, Education and Government customers.