Google employee John Mueller responded on Twitter to a question about the risks of the lack of support for horizontal orientation for the site when viewed on mobile devices.
“What happens if my site doesn’t support landscape mode on a mobile device? How does Google see this? I don’t think this feature will help in ranking, but user experience matters, right?” asked the webmaster.
According to Mueller, it doesn’t matter from the SEO point of view. It is important for Google that the page is optimized for mobile devices, but if the site does not support landscape orientation on smartphones, then this is not a big deal from the point of view of the search engine.