“Mx” the gender-neutral honorific
Solution 1:
In the UK it is almost universally an option to use either 'Mx' or no honorific. Most people will miss this, (no pun intended) because they wouldn't ask about it, however, only once in the last 2 years have I met with a situation where I had to put down a gendered honorific. This is possibly because most organisations that ask for your honorific tend to be of a more formal nature, and so are well thought out.
Places that use 'Mx' have included banks, train & flight companies, loyalty cards, utility companies and components of the NHS.
Not all organisations deal with this issue using 'Mx' though. As I mentioned earlier, some organisations are happy to simply omit honorifics, both my Internet provider and my place of work deal with it this way. As I am only drawing from personal experience, I don't know which is the more common approach.
As best I know, the only significant place where you still are compelled to identify your gender is your passport. Although the document uses the label sex, it probably means gender as a CGR is what's required to change it. A UK passport does not record an honorific, so it does not really count in this question.
As for when I first saw it, I can't honestly remember. The oldest document I can find it on at the moment is my rail card, which is only from 2013. The last time I remember its absence, however, is Facebook, who did not update their options until 2014.
I pronounce it mix as the few times I have heard someone else say it (when signing up for something over the phone) that is how it's been pronounced. It's an unusual thing to actually need to say.
Solution 2:
I've personally never heard Mx or seen it used, anyway according to M-W it is not clear if and when it will catch on in the U.S.
UPDATE
The honorific was added to the M-W online dictionary in September 2017
Last month, news broke that the Oxford English Dictionary added a new honorific for inclusion in their dictionaries: the gender-neutral Mx, used as a title for those who do not identify as being of a particular gender, and for those who are transgender — or, for people who simply don't want to be identified by gender.
Pronounced to sound like mix or mux, the title Mx. (which, like other honorifics, is styled without the period in British English) is used increasingly on various official forms in the UK, including driver's licenses and banking documents.
Although the earliest print evidence of Mx. is from a 1977 issue of an American magazine called Single Parent, the title has not seen much official or published use in the US. It did, however, appear twice in recent days in The New York Times: a June 4th article noted Mx. as someone's preferred honorific, and a June 5th article all about Mx. made it clear that the June 4th use was an exception. The title simply isn't familiar enough to the newspaper's readers to be fully adopted.
It's not clear whether or when Mx. will catch on in the US. The timeline for such developments can be long, as the title Ms. taught us not all that long ago. Coined in 1901, the now-commonplace Ms. wasn't fully adopted by The New York Times until 1986.
From The New York Times:
The adoption of the term in Britain has been easier because the idea of more than two genders is more accepted there than in the United States, so Mx. has more support there, said Kate Bornstein, the author of “Gender Outlaw: On Men, Women, and the Rest of Us.”
“The U.K. is ahead of the U.S. in terms of radical sexuality and gender, they’re far ahead,” the author said.
An article this week in The New York Times referred to a speaker using the honorific Mx. However, Philip B. Corbett, a Times editor who oversees the newspaper’s style manual and usage rules, called that appearance of Mx. in The Times an exception. “I don’t think we’re likely to adopt Mx. in the near future,” he said. “It remains too unfamiliar to most people, and it’s not clear when or if it will emerge as a widely adopted term.”
Linguistic experts say it is harder to change usage habits of words uttered frequently in speech, such as “she” and “he.” But a realignment in honorifics may be more quickly achieved because courtesy titles are less often spoken than written, like in the completion and mailing of government, health care and financial documents, as well as in newspapers and other media publications.
Katherine C. Martin, the head of United States dictionaries for the Oxford University Press, said that she and her colleagues were surprised by the huge, mostly positive reaction to online reports regarding the incorporation of the title. She said Mx. was on the “new-words watch list” for Ox