Man pages matching name stringprep_utf8_nfkc_normalize in section 3:

stringprep_utf8_nfkc_normalize.3.en	debian 10 / libidn11-dev
stringprep_utf8_nfkc_normalize.3.en	debian 11 / libidn11-dev
stringprep_utf8_nfkc_normalize.3.en	debian 12 / libidn-dev