On 09.02.2018 Google Safebrowsing marked the OptiFine website as not safe due to problems with some of the ads displayed on the site.
All ads were removed and the site was submitted for a review.
On 12.02.2018 Google removed the warning and marked the site as safe again.
The ads will remain disabled until we find exactly what was the cause of the problem.
The website has not been compromised, the downloads are safe and no user data has been affected. We take the security of OptiFine and the website very seriously.