Man pages matching name stringprep_utf8_to_locale in section 3:

stringprep_utf8_to_locale.3.en	debian 10 / libidn11-dev
stringprep_utf8_to_locale.3.en	debian 11 / libidn11-dev
stringprep_utf8_to_locale.3.en	debian 12 / libidn-dev