Good question. This is my personal view so it might not represent general opinion.
I think it depends on whether that was intentional or not. Just like misgendereing, someone referring me by my deadname isn’t a big deal if it was a mistake. It would take some time to associate me with my chosen name so it’s understandable.
However, if it was intentional (or more likely, seems like intentional), that person is not acknowledging my new gender and chosen name, and that is disrespectful or even rude.
I’m trying think of relatable experience for cis folks. Say u r in 20s and some extended family member calls u by ur old nickname when u were really young. Probably there’s no bad intention in this case, but u might get this uncomfortable feeling that
‘that (nickname) does not describe me any more. I’ve grown out of that (nickname)’
That emotion could be similar to what deadnaming feels like for trans folks. Of course u have to amplify that negative emotion by like 1000x.

















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