Transgender people have clearly existed for as far back as we can see in human existence. It’s a common but minority human trait.
We can call any trait we like an illness if we disapprove of the trait.
But all that does is render judgment. The fact is that transgender people are ordinary and harmless, to themselves and others. Labeling transgender people as ill accomplishes nothing but hurting transgender people and the people who love them.
People ought to ask themselves why they would want to do that.