The POMS page is still up however, which states that no evidence is required to change your gender marker there.
https://secure.ssa.gov/poms.nsf/lnx/0110212200
https://secure.ssa.gov/poms.nsf/lnx/0110212200
Comments
“Request for non-binary change (neither male nor female): For requests to update the NUMIDENT with a non-binary sex marker, such as an X, explain that we can only accept a sex marker of male or female, using the following script;”
Im sure that that isn't the case NOW.