For OpenSSH, its man page is the canonical documentation. According to ssh(1) - OpenBSD manual pages,
/etc/ssh/ssh_known_hosts is the official path for the “systemwide list of known host keys”. This applies to all releases of OpenSSH and is not specific to any one distribution – or OS for that matter.
SSH.COM is the website for SSH Communications Security, a commercial company set up by the original designer of the SSH protocol and writer of the first implementation. It has no direct relationship with the OpenSSH project which is maintained by a group of OpenBSD developers based on a fork of an early release of the original SSH program.
I don’t know where SSH Communications Security got
/etc/ssh/known_hosts from (presumably their own or the original implementation) but as a third-party source of information, it is not relevant to users of OpenSSH. Out of interest, I checked the earliest version (rev 1.1) of the OpenSSH man page and it had historically been using yet another (slightly different) path,