Man pages matching name x509_name_get_index_by_nid in section 3ssl:

X509_NAME_get_index_by_NID.3ssl.en	debian 10 / libssl-doc
X509_NAME_get_index_by_NID.3ssl.en	debian 11 / libssl-doc
X509_NAME_get_index_by_NID.3ssl.en	debian 12 / libssl-doc