I've created my global page on meta.wikimedia.beta.wmflabs.org. When I opt it out on another wiki, it is still displayed there. I expected it not to be wisible.
FYI, create a central user page on meta.wikimedia.beta.wmflabs.org displays that message before saving:
Error: Warning: This action has been automatically identified as harmful. Unconstructive actions will be quickly reverted, and egregious or repeated unconstructive editing will result in your account or IP address being blocked. If you believe this action to be constructive, you may submit it again to confirm it. A brief description of the abuse rule which your action matched is: <div class="mw-parser-output"><p>Log possible Spam edits by new users II </p></div>
Thank you for working on it!