What's not appropriate, is referring to the person that exists today, as Ellen Page.
Even if you are not transgender, just imagine changing your name, telling everyone about it, and someone would come along insisting on calling you using your former name, practicly mocking, belittling you and your choice. Now imagine them doing the same to your entire identity.
Or a daughter who dislikes her father so much that, upon reaching adulthood, changes her surname because she wants to be completely disassociated from him. (In the case I'm thinking of, she said something along the lines "he wasn't my father, he was the sperm donor" - he left when she was a few years old.)
Personally, I understood the idea first through Malcolm X, who rejected "the white slavemaster name of 'Little'" he was given at birth. And the respect Howard Cosell had to refer to Muhammad Ali by Ali's chosen name, while most others used and even taunted him with his old one.
It isn't.
Talking about someone's dead name is not offensive.
Referring to someone using their deadname today is. For example, saying "Ellen Page was really good in the latest season of The Umbrella Academy" is offensive because at that point, they were identifying as Elliot.
In the case of someone not being notable before a transition, talk of their deadname is offensive because it is clearly being done in bad faith. If they were transitioned before they became a notable person, why the obsession with knowing their deadname? There's really only one reason: Because you want to call them that as an "own" against transgender people.
Why is it offensive to say that my celebrity crush used to be Ellen Page (until she came out as lesbian)?