As part of their pricing switch a few years ago, Google committed to a 3 year depcreation period for App Engine: http://googleappengine.blogspot.com/2011/05/year-ahead-for-g...

"For App Engine, leaving Preview will include providing all paid users a 99.95% uptime service level agreement, operational and developer support, billing via invoice, a new Terms of Service agreement geared towards businesses, and a new, easier to understand usage-based pricing structure for App Engine that is in line with the value App Engine provides. It will also reaffirm our deprecation policy whereby we will support deprecated versions of product APIs for 3 years, allowing applications written to prior API specifications to continue to function."

A quick comparison of the TOS shows that Compute Engine doesn't have the same policy, but IANAL, so I could be missing something.