As far as I know, there is no commonly used replacement for “Mr.” or Ms." Some have proposed “Mx.” but this is not yet standard.
For letters where you believe that level of formality is appropriate (which can include emails in some cases), there are other options. If the person has a title, you can use that. And I do believe there is a move to actually use full names, with no honorific at the start, e.g. “Dear Pat Mathis;”.
For most emails, however, I just respond without any opening greeting at all.
As someone who taught English composition at the college level for many years, I must add that my weakest students, long before there was any issue of gender bias in pronoun use, would use “they” as a substitute for “he/she” because they weren’t sure of the appropriate form to use. Laziness, in other words. These students weren’t seeking neutrality --they wouldn’t know gender bias if it bit them in the ass. It was only in the last few years that these students would claim that “they” was the correct pronoun when it clearly was inappropriate because they were transgender or some other thing that was a newly emerged term of art. Not every one, of course, but I felt it was about the same number of students, maybe a little bit higher now, as in the past who simply couldn’t be troubled to master the niceties of a sentence like “Students may leave the room to go individually to his or her locker but only with permission and only one at a time.” They always wanted to write “…individually to their locker…” although the context was clear that only one student of either sex would go to his or her locker. When I corrected such sentences, I got such grief (from my fellow instructors as well as from students) I threw up my hands and said “Oh, hell, write any way you please.” And promptly retired from teaching grammar and usage.
Back last century I was writing documents that frequently seemed to indicate what I thought was the ugly he/she. For example: “If the driver of a company vehicle is involved in a collision then he/she should…”
Substituting “they” looked and read much better in my opinion then and now. We had no concerns at that time about those people whose genders may be indeterminate.
I don’t feel that using “they” as gender neutral is particularly new – certainly not something that is now “becoming” standard. There are, of course, all the historical cites to its usage, but it was still considered non-standard in formal writing at least through the 90s. I remember many college textbooks that used the clunky “he/her” or “he or her” construction or, even worse, alternated the pronouns “he” and “she” from paragraph to paragraph, which drove me absolutely batty. Otherwise, we were told to recast the sentence into the plural, if possible, if we wanted to avoid “he/she” type constructions and maintain gender neutrality.
One possible set of data is to observe the construction “he or she” in the written corpus. It’s not going to be a perfect proxy for “they” substitutions, but I’d hypothesize that it gives us a quick and dirty idea of when “he or she” was going out of style and replaced by some construction. And my guess is that construction would most likely be “they/them.”
In that graph, we see “he or she” gaining traction in the late 60s, presumably replacing “he” as a gender-neutral construction, and peaking in 2002, before falling rapidly to 1982 levels of popularity.
I would also think the written corpus lags behind popular usage, but with that data I would observe that somewhere in the late 90s/early 00s is when at least “he or she” began losing popularity, and hypothesize when “they” gained traction as a more accepted replacement. The usage of 'they" as a substitute is not proven by that graph – we would need either hand annotations about the referent or some machine-learning driven approach to label whether referents of the pronoun “they” in the English corpus post-2002 began to refer more and more to singular subjects of unknown gender, but I think it is a reasonable conjecture.
Similarly, I have noted an uptick in usage of “they” through the 21st century, but its usage is about two orders of magnitude more common than “he or she,” so it’s a bit difficult to parse out how much of that usage is attributable to the decline of “he or she.”
This happens to me all the time. I work with a lot of folks in India, China, and Japan, and I’m not yet well versed enough on names to know which are generally male or female. I tend to use whatever name they signed off with and avoid pronouns until I know for sure.
Chasing Amy came out in 1997… discussing Alyssa’s mysterious new flame
DALIA Come on, Alyss - Hoboken Hussy or what?
ALYSSA For your information, they don’t have big hair or wear acid wash. (goes back to painting) They’re from my home town.
DALIA Why are you playing the pronoun game?
ALYSSA What? What are you talking about? I’m not even.
DALIA You are. “I met someone.” "We have a great time. “They’re from my home town.” Doesn’t this tube of wonderful have a name!
ALYSSA (beat) Holden.
All four Girls stare at Alyssa, a bit horrified. She stops painting.
I assume it’s a piece of screenwriter humour that “tube of wonderful” is an equally gender-ambiguous reference.
I suppose it’s a bit less confusing using “they” than some of the latin languages where every pronoun has a gender associated with it. An old-timer where I worked suggested the rule in English grammar was similar to French, Spanish, etc. “The male embraces the female where desirable.”
But I have to agree with the gist of the thread, which is that “they” and its variants has been a perfectly good ambiguous singular pronoun for pretty much always.
Was there similar debate when “you” became the singular pronoun too, replacing “thee”?
As a professional copy editor, long before" they/them" became the preferred pronouns for non-binary folks, I found that the simplest way to avoid awkward “he or she” or “he/she” constructions was simply to recast the sentence to make the subjects plural:
“If drivers of company vehicles are involved in collisions then they should…”
You did not become a singular pronoun and replace thee. They coexisted as singular pronouns. You is the formal form, used when addressing your betters, and thee is the informal form, used when speaking down to a peasant or between close especially close friends. Over time, the informal version dropped out of usage. Guess we all just got too big for our britches, demanding to be addressed as gentry.