dead-name

/D EH1 D N EY0 M/
noun
  1. 1

    A previous name of a person (especially a transgender person) who has since changed their name.

verb
  1. 1

    To refer to someone (especially a transgender person) who has changed names by a previous name.

Source: https://en.wiktionary.org/wiki/dead-name, https://en.wiktionary.org/wiki/deadname