Man pages matching name curlopt_conv_from_utf8_function in section 3:

CURLOPT_CONV_FROM_UTF8_FUNCTION.3.en	debian 10 / libcurl4-doc
CURLOPT_CONV_FROM_UTF8_FUNCTION.3.en	debian 11 / libcurl4-doc
CURLOPT_CONV_FROM_UTF8_FUNCTION.3.en	debian 12 / libcurl4-doc