I'm gonna be thinking about this all day
If you deadname someone in the death note it kills you
In one of the Death Note live action movies (I think it was Light Up the New World), a man changes his name after harboring guilt about past crimes and trying to atone (not even answering to that name). He's only killed after his new name gets put in the book, since he shed his old name.
Man wasn't trans, but if the logic applied to him, logic should apply to trans people too!
This rule implies that birth names are not necessarily the names given to you at birth.
That means you can change your name or consider yourself to have a new name and the death note recognizes it.
Death Note is an ally.
Diversity win! this Death Note kills trans people but doesn’t deadname them!
I had to look this up to double check, but there is a rule that if the owner accidentally misspelled a person's name 4 times, that person is immune to the Death Note, but if the owner purposefully misspelled a person's name 4 tines, the owner will die.
So, theoretically, if the owner knew a trans person's deadname and knew their real name, but decided to be transphobic and wrote their deadname instead of their real name 4 times, then the owner would die.