Many questions get closed under the rubric "Language-specific grammar and usage questions are off-topic unless primarily concerned with linguistics rather than usage." My understanding of this close reason has always been that it should apply to questions related to what is correct or good usage, i.e. questions taking a prescriptive perspective.
In practice, I find it is applied far more liberally, such as for this recent question. This is a question a well-known phonetician has written a blog entry about. Any English linguistic could write a paper assessing the frequency, social conditioning and phonological analysis of this phenomenon.
Linguistics isn't only about general linguistics but also analysing single languages and their systems. A lot of research in linguistics is conducted in university departments dedicated to specific languages, i.e. English/French/Spanish/German departments.