Twitter wonks noticed this happening moments after Trump was sworn in. Via Esquire:
Donald Trump was just sworn in as the 45th President of the United States. Just gonna leave that there for a moment. As part of the presidential transition, Trump’s team assumes control of WhiteHouse.gov. Before today, the “Issues” tab of the website had a section devoted to climate change, the existential threat posed to our species that a scientific consensus holds is caused by human activity. That section is completely gone with the website revamp, along with any mention of climate change whatsoever. The Civil Rights section is also missing. And the LGBT section.
This is the new issues page.