Nat’s gone and done the research, it must’ve taken ages, and it is really interesting and helpful: When was the Mx gender-inclusive title created?
The Google Groups Usenet archive extends back to 1981 and the earliest example I found of someone suggesting and using Mx (albeit as a one off) was in July 1982 on the newsgroup net.nlang during one of their frequent discussions of gender neutral pronouns (intended to be used to refer to anyone without giving their gender) and nonsexist gender neutral language (such as ‘chairperson’ and ‘womyn’)
Nat’s article goes on to document more mentions online, and the first serious mention of its practical application in 1998:
Subject: Re: Vegetarianism and B12 deficiency (was Re: Organic GE)
Newsgroups: uk.misc,soc.culture.british,rec.food.veg,uk.politics.animals, uk.people.teens,soc.culture.scottish,uk.environment
Occasionally I have used the title ‘Mx’ before my name, with the idea
that it leaves in question whether I a woman or a man or somethinng in
between and gives no idea of my maritial status.
I love these little insights into gender and linguistic history. Big appreciation to Nat! I’ve updated the Mx PDF to include this new information (and a couple more examples of its use, by the University of Birmingham and the Institute of Physics), so be sure to download the new version for emailing to stubborn people. Direct download / Scribd
— In response to the question, “What does gender mean in the absence of gender roles?”