Message from Adobe:
"To make accessing a design spec more seamless, effective as of March 21, 2018, viewing a published design spec will no longer require a sign-in with an Adobe ID. All existing design spec URLs remain the same and are still public links, meaning that anyone who has the link is able to view it. Any new design spec that you publish will also no longer require a sign-in with an Adobe ID."
That's a quote from: Make Design Spec publicly accessible – Adobe XD Feedback : Feature Requests & Bugs