Wishfull thinking, fantasy, passing on of false information, this is was makes up this unfounded subject. It seems pretty ridiculous, to think it could be reality. There’s nothing in the real world, of girls entering into this, that can be true. I point to one thing (there are many), in particular to say it’s pure fantasy. There are stores, big and small, that carry and cater to Confirmation dress (clothing), both boys & girls. If it were true, these stores, or departments with in stores, would have such articles available for purchase. THEY DO NOT! No store that sells Confirmation clothing, has diapers and plastic pants, for such a purpose, never have.
I understand, and enjoy such notions, and or fantasies, just as much as I understand, Bugs Bunny was not a real living rabbit.
I received my Confirmation, watched my sister receive hers, plus many family & friends, and never once has there been a diaper involved. Never heard anyone ask about such, or seek advice on such a practice.
So my advice, go ahead and have your fantasy, talk about such fantasy, write a story about it, but don’t think for a moment it’s a real thing, or ever has been. Except for, what someone already mentioned, it could be a girl is in a diaper, but it would be because she has wetting issues.