For tasks like deep category search (and possibly others, e.g. Wikidata maintenance) we need SPARQL client to communicate with SPARQL endpoints. Since it is going to be used across different extensions, it makes sense to make it core service.
Well, this client should probably be in a library soon / already anyway?
Said library should contain the docs, It might even be worth marking it for internal use until we put it in a library.
I must have misunderstood the newsletter sent out to the public. I thought that is is about a user facing something. Anyways, documenting stuff is always good and I know from experience that this is painful.