I agree about warning about hazards. This isn't a hazard though. A black person in a former white only bathroom isn't a hazard and shouldn't, and didn't, erect a sign warning people who were used to that. And I also agree that this isn't like trigger warnings, where exposure to a stimulus can cause real harm to someone suffering from PTSD. A human just existing in a public space isn't harmful.
Would they feel any better if this person, who is a person born female, came into the women's bathroom? A transphobe would call this person a female even though they're a transman, but it wouldn't reduce the 'shock value' if they met them in a bathroom.