OSX 10.X App Compatibility Legality - Parallels
To provide an insight into the API level changes between OS X versions, Apple's What's New in OS X developer document is useful.

The deprecated framework and function lists can be extensive between 10.x releases:


  Deprecated Frameworks and APIs
  
  Periodically, Apple adds deprecation macros to APIs to indicate that those APIs should no longer be used in active development. When a deprecation occurs, it is not an immediate end of life for the specified API. Instead, it is the beginning of a grace period for transitioning from that API and to newer and more modern replacements. Deprecated APIs typically remain present and usable in the system for a reasonable time past the release in which they were deprecated. However, active development on them ceases, and the APIs receive only minor changes to accommodate security patches or to fix other critical bugs. Deprecated APIs may be removed entirely from a future version of the operating system.
  
  As a developer, avoid using deprecated APIs in your code as soon as possible. At a minimum, new code you write should never use deprecated APIs. And if your existing code uses deprecated APIs, update that code as soon as possible. Fortunately, the compiler generates warnings whenever it spots the use of a deprecated API in your code. You can use those warnings to track down and remove all references to those APIs.