And do note that putting it just anywhere in your httpd.conf file is not sufficient. It needs to apply to the https requests in the portion of apache* being used as a reverse proxy, which generally means it is best off in the config for your https virtual host. Bottom line: the additional header needs to be applied to every request that is passed to Check_MK.
And yes, the instructions at Operating Checkmk with HTTPS are still broken.
*I can’t think of any particular reason you can’t use any reverse proxy in front of Check_MK, though apache is handy given that it’s already installed for Check_MK to use.