It may not be the first

Correct.
Am riding drop-bar touring mountain bikes for almost 15 years now and not looking back.
(Salsa Fargo, steel version, have currently two active at home - one for good weather, one for bad conditions).

https://theradavist.com/mike-varleys-dropping-in-article-from-1991

I think drop bars on MTBs can be a cool idea, at some point I thought about doing a conversion before getting my first mtb, but 'm not sure I'd like that specific aero-y drop bar in the picture