Q:

Is it appropriate to answer a question that is tagged to a specific language version while my answer uses a newer API? 

Imagine a situation:
there is a good question that is tagged to a specific language version (for example: java, java-8) with good answers.  At the same time, there is a new API method in the newer version of the language (for example, in java-9), that certainly can be used in order to solve a problem! 
Is it appropriate from my side to answer the question with an example that uses a newer API? 
I didn't find any similar questions and I think it is a good canonical question. 

A:

No.  Yes.  Maybe. 
There is everything wrong with providing an answer that uses a functionality only available in a newer API, if the question is tagged with a specific version. 
People (are supposed to) use version-specific tags because they're stuck with that version, or because they have a question about functionality introduced with that version.  
So yeah you can come and barge in and answer "Using version N+M you can foo the bar", but that won't help the OP nor later visitors who also are stuck on version N.
So if you must, you should also mention very explicitly that you don't know how, or that you do know how you can't, do X with version N, but that if the reader can upgrade to version N+M, they could do it in the way you then describe in your answer.  
When a new language, framework or library version introduces commonly discussed or requested features, that's better posted as an answer to a generic, version-unspecific question asking about said feature. 
See for example C# variables in strings

