Man pages matching name stringprep_unichar_to_utf8 in section 3:

stringprep_unichar_to_utf8.3.en	debian 10 / libidn11-dev
stringprep_unichar_to_utf8.3.en	debian 11 / libidn11-dev
stringprep_unichar_to_utf8.3.en	debian 12 / libidn-dev