To the best of my knowledge, they are still made in Nazareth PA, in the same shop as all of the D-28s and D-41s etc - the difference is mainly in the level of ornamentation (ie: on a D-15 basically none - not even binding around the top from memory), and on some of the least expensive models I believe the back and sides may be laminated (plywood)
If you look inside the soundhole of almost any modern Martin it has "Use Light or medium
gauge strings only" stamped on a brace - I have had a 000-16C for about ten years now - a somewhat battered little guitar which saw far more abuse on stage than she deserved, but she still sounds lovely.