detransitioner
Noun, for someone who is detransitioning, ie halting or undoing gender transition. A detransitioner might or might not continue to identify as a member of the trans "umbrella".
I'm a detransitioner and proud.
Political Detransitioner
A person who has detransitioned and now takes part in advocating for anti-transgender legislation (typically gender-affirming care restrictions) or against transgender movements and rights. Sometimes this is strictly for political reasonings or revulsion against gender-affirming care providers or transgender people.
A political detransitioner came to testify in support of an anti-gender-affirming care bill.