I would caution against making a sweeping decision that applies to all language-version tags.
Some languages, like java, are conservative, and introduce new features with very few incompatible changes between major versions. It makes sense to abolish java-8.
Other languages are more innovative. For example, python has introduced breaking changes from Python 2 to 3. Sometimes, it is immediately clear from context whether code targets Python 2 or Python 3, sometimes it is not obvious, and some code runs flawlessly on both. Therefore, the python-2.7 and python3 tags are situationally useful, if the author wants to specify the language version. Therefore, for Python at least, I think that the language-version tags should be used at the discretion of the author or editor of each question — the tags should be neither mandatory, synonymized, nor merged.