Yes, but the Death Watch didn't have a thing about not taking off their helmets when we saw them in the Clone Wars. And none of the Mandalorians in the Rebels had an issue with taking their helmets off.
I imagine it was developed as a post-purge survival technique, along with the whole "only one allowed out at a time" rule. I would also be willing to bet that there are other Mandalorian "coverts" hidden around the galaxy.
I would also love to know if the Armorer is someone we've seen before or just a random Mandalorian. I doubt she's either Bo-Katan or Sabine. She could theoretically be Sabine's mother, but that seems unlikely as well.