We don't really have one tho I think we loosely follow Semantic Versioning. In Android-land we recently merged in CordovaView which has an API change which will break many plugins. In Semantic Versioning reality we are completely justified to kill an API in 2.x which would make a great deal of pain for plugin authors at that time. Major release, break all the things, seems bad.
Anyhow, I'm thinking we look to create a policy that is time based. We make a change, its got 6 months of deprecation notices, and then its gone, regardless of version number. Thoughts?
