Should we have a dedicated tag for Pathfinder Unchained in addition to a tag for regular Pathfinder? Or are the two systems close enough that a separate tag isn't needed?
On RPG.SE tags are emergent not prescribed. If there are enough Unchained questions, and if a tag becomes necessary to distinguish them, then we might have one. In general we avoid subsystem fiddly tags unless there is a critical mass that makes them worth it. So "no, unless we get a batch of questions that use it."
It's also not a centralized decision. If you have more than a token amount of rep, you can add a new tag to your own question. If that tag gets used on more than one question, it'll stay around (solo tags get reaped after a time period). And lo, a new tag is born. Just because there is a tag doesn't mean someone has to use it - even if a pathfinder-unchained tag were to be created, someone could ask an unchained question without it just fine.
More specifically to this case, we tend to look down on creating tags for one specific sourcebook for one specific game. There's thousands of games and many of them have books full of rules and/or optional rules, we prefer the "use your words" approach and not having 200 pathfinder tags, 50 shadowrun tags, etc. each with a specific product name in them.