We have a unix tag but because the entire site is effectively about Unix this tag would get abused and become pointless. Therefore it's restricted to only historical aspects of Unix. There are a few Q&A's that use it (currently 9 Q's).
If you take a look at the info about the topic it explains its purpose and role:
(In more detail: Stack Exchange developers decided in 2011 that the names of the individual Stack Exchange sites should be blacklisted as tags. So initially both unix and linux were blacklisted. People objected to blacklisting linux at https://unix.meta.stackexchange.com/a/565/5132 and the blacklisting was removed.)
