Deadnaming is an act of calling someone by a name they no longer identify or resonate with. Typically this is done when the birth name or the former name of trans or non binary person is used without their consent, usually in an attempt to be cause harm. Deadnaming is hurtful and it invalidates an individual identity.
Sources:
https://www.medicalnewstoday.com/articles/deadnaming
https://www.verywellmind.com/what-is-deadnaming-and-why-is-it-harmful-5188575
https://health.clevelandclinic.org/deadnaming/
https://www.healthline.com/health/transgender/deadnaming#impact