Man pages matching name idn2_to_unicode_4z4z in section 3:

idn2_to_unicode_4z4z.3.en	debian 10 / libidn2-dev
idn2_to_unicode_4z4z.3.en	debian 11 / libidn2-dev
idn2_to_unicode_4z4z.3.en	debian 12 / libidn2-dev