"Master", in the MA context, is a short form of "Master Instructor", and is a term of respect that denotes the person's achievement. In ITF TKD, "Master" ranks begin at VII Dan, and therefore there are rather fewer of them than there are in other organizations and/or styles that consider lower degrees of BB to be masters. When I call a VII or higher Dan "Master", I am recognizing their rank and demonstrated skill, both in the art itself and in regards to their teaching skill - rather along the lines of calling a physician or dentist "doctor", in recognition of the skills and knowledge required to attain that rank, or calling a college instructor "professor". I do not believe, for myself, that using the term "Master" in a martial arts context connotes any form of that person being my master in the religious or social (e.g. slavery) sense. As in other threads on this topic, "master" is a term with multiple meanings, and people often object to the use of "master" based on other meanings than the one intended in this context.
Do I know people who introduce themselves as "master so-and-so" in MA contexts? Yes, I do - and I find it to be appropriate in that context. Do I know people who introduce themselves as "master so-and-so" out of MA contexts? Not at present, no - and I would find it inappropriate for them to do so. For example, my father has a Ph.D. in Library Science, and is, therefore, qualified to use the term "doctor" as a term of address - but he has chosen not to do so outside the academic world (he's a college professor) because he finds it to be both inappropriate and confusing - from the time I was a child he introduced himself to my friends as "Mister" not "Doctor", because he didn't want to mislead them into thinking he was a medical doctor.
Do I know people who introduce themselves as "master so-and-so" in MA contexts? Yes, I do - and I find it to be appropriate in that context. Do I know people who introduce themselves as "master so-and-so" out of MA contexts? Not at present, no - and I would find it inappropriate for them to do so. For example, my father has a Ph.D. in Library Science, and is, therefore, qualified to use the term "doctor" as a term of address - but he has chosen not to do so outside the academic world (he's a college professor) because he finds it to be both inappropriate and confusing - from the time I was a child he introduced himself to my friends as "Mister" not "Doctor", because he didn't want to mislead them into thinking he was a medical doctor.