When a question or problem arise for a user, is it allowed to reference to a plugin that solves or fulfils the requirements set in the question?
I think the current state of things is that asking for a plugin is a bad question, but answering with a plugin is a passable answer.
Still the considerations of "link only" answers apply. Personally I try to formulate it along the following lines: "There is a [plugin] that does [this|something similar], in a [following way], and you might consider using it or studying [source here] as a prior art."
Note that this works best for public plugins. Such answers for commercial plugins are almost always seen and treated as spam.