Is “-th” still a productive suffix in English?

The short answer: yes, it is productive, because you can create words using this suffix that have never been heard before, such as the two-trillion-and-sixteenth coin in Scrooge McDuck's Money Bin. From Wikipedia:

A productive grammatical process defines an open class, one which admits new words or forms. Non-productive grammatical processes may be seen as operative within closed classes, but only previously formed and learned structures show the results of those processes. (Italics mine.)

The point of productivity is not that you can add a suffix to any word you please, but that it can be added to some words to create new words. Only if it cannot be added to any words in order to create new words is it said to be unproductive.