Sadly, that "Press Esc to exit full-screen mode" message is controlled by YouTube/Chromium itself, not Opera GX. There’s no built-in setting to disable it. A workaround is to use a browser extension or userscript (like Stylus + custom CSS/JS) to hide that notification. Otherwise, we’re stuck with it for now.
Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you have been placed in read-only mode.
Please download a browser that supports JavaScript, or enable it if it's disabled (i.e. NoScript).