This stems from an issue with the official docs, but 'tag sets' are a read preference idea whereas the string variant of w (excluding "majority") on write concern refers to a write tag or custom write concern mode. So the function on writeconcern.Option should be something like WTag(tag string) or WCustom(tag string).

I believe the docs team is updating the write concern page, so this fix could be deferred to use whatever terminology they decide upon.

Deprecating the old WTagSet and omitting it from the documentation may have to be a part of this fix.