Why is this RC eligible
Because it only changes docs.
As far as I can tell, since site branding was moved into block--system-branding-block.html.twig, the site branding variables site_name, site_slogan and logo are no longer available to use in page.html.twig. However, they are still included in the docblock in the core page.html.twig and each of the three core themes that override it (classy, bartik and seven).
Unless I've missed something and these variables can still be accessed from these templates, I would like to remove the references to them from the docblock text to avoid confusion.