If there are major and possibly non-backwards compatible differences between a new version of a language and its previous versions, then I think we should keep the tag.
For example, lets look at C++.
Bjarne Stroustrup said that C++11 feels like a new language.
I personally have found the c++11 tag to be useful.
My answers can sometimes vary depending on whether or not the OP has a C++11 compiler.
It also helps filter questions. I know I learned a lot about C++11's new features just by looking up random questions with this tag.
I feel like @Mat'sMug's answer is probably best for the long run, but having these versioned tags temporarily can be very useful.