Man pages matching name idn2_register_ul in section 3:

idn2_register_ul.3.en	debian 10 / libidn2-dev
idn2_register_ul.3.en	debian 11 / libidn2-dev
idn2_register_ul.3.en	debian 12 / libidn2-dev
idn2_register_ul.3.en	debian 13 / libidn2-dev