This market resolves as YES if there is an elected official at the state or federal level in the US who is publicly detransitioning or publicly has detransitioned before 2029. This includes both those in and out of office. If you already know of an elected official who is openly detrans and provide evidence of their existence from local or national news, then this market resolves as YES. Personal communications (say on social networks) confirming the person as detrans do not qualify. If there is not a detrans elected official before 2029, then this market resolves as NO.
Betting YES:
A vocally transphobic detransitioner (as opposed to a non-transphobic one; most detransitioners aren't transphobic) could definitely win a Republican primary.
Alternatively, a non-transphobic detransitioner could win a Democratic primary; though it's unlikely because detransitioners are less than 8% of less than 5% of the population - and those estimates are both on the higher end. Using lower estimates for both gets you less than 1% of 1%, or less than 1 in 10,000 people.
Insurance because if a transphobic detransitioner wins a Republican primary, let alone a general election, it's going to make online transphobes even more vocally annoying than they already are.