Google's John Mueller said that "there is no effect in regard to SEO for Google" when it comes to if a site or page supports landscape orientation. Yes, it does matter if the page is mobile-friendly and has content on mobile-first indexing, but if you orient your phone sideways, it doesn't matter.
John said this on Twitter:
There is no effect in regard to SEO for Google.
— 🍌 John 🍌 (@JohnMu) June 29, 2020
I never heard anyone ask this before, so this was an interesting question for me.
Do you think Google should care about a page that does not render great when you turn your phone sideways?
Forum discussion at Twitter.