a simple solution for a splash, landing, or entry page, that does not require setting cookies, is to first create a new folder “YOURBLOG” and move all the files from the top level into that folder.
then create a new index page at the root of the site with your warning and link that to “YOURBLOG” (relative link).
in the WP admin, just go to options and change the URLs to your site from
“https://www.YOURSITE.com”, to
“https://www.YOURSITE.com/YOURBLOG”
viola
Phil