In latest Google Chrome, the store throws a warning for insecure interactions on the page.

Mixed Content: The page at 'https://store.wikimedia.org/' was loaded over a secure connection, but contains a form which targets an insecure endpoint 'http://wikimediafoundation.us4.list-manage2.com/subscribe/post?u=438f0ff3a0fa6a75eeaea7498&id=906a788d5c'.

This endpoint should be made available over a secure connection, or the footer should be changed to not embed a form but merely link to the external form page by anchor link.