messe parent
AFAIK, neither OpenBSD nor NetBSD has sendfile. On FreeBSD, I think you're correct regarding it being file-to-socket only.
Indeed, if I'm not mistaken Netflix at least used to use (and commit to kernel) FreeBSD on content servers because of its superior sendfile performance
Not only that, but they even contributed patches to allow the FreeBSD kernel to handle the TLS part of SSL_sendfile as well[1].