jQuery isn't a language. It doesn't have a syntax of its own. So a question about how to implement a jQuery plugin is a question about jQuery. Sure lots of coders could get by quite happily using jQuery without ever writing a plugin, but still.
Having said that, I would expect the term "plugin" to appear in the question title and/or in the first sentence of the question body - thus making that information visible from the question list so that people not interested in plugins can ignore such questions.
And having said that, I have no objection to a
jquery-plugins tag. For a "non-programming" question along the lines of "Is there a jQuery plugin that does x?" using a
jquery-plugins tag but not the
jquery tag makes sense.
If anything, I think the
jquery but not with