Man pages matching name idna_to_unicode_4z4z in section 3:

idna_to_unicode_4z4z.3.en	debian 10 / libidn11-dev
idna_to_unicode_4z4z.3.en	debian 11 / libidn11-dev
idna_to_unicode_4z4z.3.en	debian 12 / libidn-dev