I have heard of Mx.
On forms - personally I try to not select a title, and although my given name is fairly androgynous I usually just put my first initial. On the awful automated forms that insist on a title, I have been using Dr (for many years) or if it's available Mx.
I believe a persons biological sex (and sexuality) is their own business. At times a medical professional may have a need to know this information, but only if the persons biological sex has an impact on diagnosis/treatment.
If everyone is supposed to treat everyone else equally, then gender/sex/sexuality/marital status/parental status/religion/race/etc shouldn't matter, if these things shouldn't matter why do you need to know, and why should people be expected to 'declare by default' by their title?
I don't consider myself 'uber-woke' or whatever a pp said. I'm no spring chicken and I've held these views from a young age. I'm a very private person and my default stance when asked a question is "why do you need to know?" not aggressively or argumentatively, and I'm very happy for you to explain if you have a legitimate reason.
Not too long ago a woman who was married would be Mrs husband-first-name husband-surname (ie Mrs Jonathan Smith) I know few women who would be happy to accept that now.
I used to take customer info over the phone, and there is a growing number of people confused re Ms/Miss/Mrs. I had to start clarifying as people would say their title was miss because they were reading m-r-s as a word, not knowing the standard pronunciation is missiz/missus (it's actually short for mistress and used to be used for both married and unmarried women!), others were saying mis-s for Miss, which of course sounds like missiz, originally I'd have said this group was predominantly made up of those who spoke English as a second language, yet more and more young people seem equally confused.
And of course there is still a shocking number of people who think Ms means either divorced or lesbian.
As all these titles end up with negative stereotypes, surely the best way forward is to do away with all the old ones altogether? (Replacing them with a single one like Mx clearly isn't going to work while prejudiced people insist on making assumptions about the people who choose to use them!)
A letter addressed to 'first initial, surname' will reach me just as well as one addressed 'title, first initial, surname' and if you want to be formal while speaking to someone use firstname+surname.
I couldn't tell you the title for most of the 350ish people I currently work with, but we get by just fine using our names.