Ugh. So hard to tell what subforum you're in.. I literally didn't realize this was a question placed in the prior editions area. Mea culpa.
I could have sworn 5e had this same rule, but yes I'm not turning it up. Still, expansion splatbooks have on occasion just injected new specializations into the mix, so there's no reason your table GM can't do it. Besides, Rule 0 trumps every other rule. So, so long as your GM is fine with your custom specialization, you're golden.