I'm aware of the form "womyn" from the 90s, but "womxn" appears to be more recent, and differentiated from "womyn" by its explicit inclusion of trans women and woman-like nonbinary people.

Edit: Wikipedia traces "womxn" back to the 1970s, but it seems unlike "womyn" it didn't get much currency in ordinary feminist usage, only finding purchase when the queer angle was added.