The Backward Compatibility is the properties of the sequence of version not of any individual version release. In Backward Compatibility, the code compiled on an older version will link with code compiled with newer ones.
This is one of the biggest issues with Scala while upgrading Scala in the deployment environment you need to re-compile your application.
If you built a program against 2.1, you couldn’t just run it against 2.3 – you’d have to recompile.
Things are improving, Scala community is very active, they are working to resolve the same and soon we can switch between minor releases.