Solaris in everyday speech means SunOS versions 5.0 and above. (Officially, it means 4.0 and above as the last BSD major release, SunOS 4, was retroactively branded Solaris 1.) The last release of SunOS 4.x was 19 years ago, and support for it ended 10 years ago. Not a single question on the site is about such old versions (I think the oldest Solaris release mentioned in a question is 8).
So we don't need a tag to mean “SunOS when it was a BSD”, and the tags “Any of the two incarnations of SunOS” and “SVR4-based SunOS” cover exactly the same question. This is increasingly unlikely to change as time progresses.
It is natural to use the tag sunos for recent releases of Solaris, since that's the name
uname -s displays.
Given the unusually wide support and lack of opposition to this meta question, I say make it a synonym.