The honorific Ms. started way back in the 17th century but fell out of use in favor of the Mrs. and Miss monikers. It was refashioned in the 20th century for professional women so they could mirror the status neutral Mr. Mx. (pronounced Mix) was coined back in 1977, but it wasn’t until 2015 when Oxford added the word to its dictionary (2016 for Merriam-Webster) that Mx. became an official title.
In the 1980s, when Geraldine Ferraro was running for vice-president, the Ms. moniker became the norm for everyone. It had previously been used for feminists or used by others when they were unsure about the marital status of the person they were addressing. Today it is used more as the standard respectful title to refer to woman as equals.
While the evolution of Ms. took centuries to become what it is today, Mx. is growing fast. Already allowed on drivers licenses and bank papers, Mx. may not be done evolving. Today it is commonly used by people who don’t wish to identify by gender. “To whom it may concern” is as long as it is cold. “Dear Mx.” is much warmer. In the would of linguistics there is room for everyone.