I'm not sure if we still need both unix and linux. Unix is a broader family of operating systems and Linux is one of them under Unix. I don't think we should just have linux since there are questions about other Unix-like operating systems.
Synonymizing these tags would be an absolutely wrong and harmful decision.
Let's look across the Stack Exchange network to see what sites like "Unix & Linux" or "Super User" say about Linux, Unix, and other operating systems:
Is Linux a Unix? Which is answered with "it depends", basically.
Is Mac OS X UNIX? Which is answered mostly with "yes".
There are other questions and answers on the topic regarding comparing various operating systems to each other and pointing out their similarities.
At the end of the day, unix may be a bit like an umbrella tag, under which you could put a handful of other operating system tags, like osx, linux, android, ios... but Linux and Unix are not exactly the same thing. Nor is OSX and Unix.
While things that work in Unix will tend to work in OS X or Linux, it's not necessarily true for the reverse.