Man pages matching name curlopt_tcp_keepidle in section 3:

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