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.
Best Answer
According to the OALD, Jew can refer to either sex, while Jewess is old-fashioned and usually offensive.
MW also agrees with the neutrality of Jew and possible offensiveness of Jewess.