It does cover backwards incompatible API changes - read the second link I provided:
>Further, Google will notify Customer at least 12 months before significantly modifying a Customer-facing Google API in a backwards-incompatible manner.
What are you referring to with forced sdk/software changes? Maybe it's the same exception I mentioned for App Engine when a previously supported language runtime goes out of support by the upstream language community?
As for the famous Google App Engine price hike everyone likes to point to, the lower price was only at a pre-general availability launch stage.
You're right that they don't make any specific guarantees that they won't hike prices on short notice, but within the scope of Google Cloud Platform features that have already reached general availability, when have they done that?