Creating an opt-in beta testing UI for new features on your website can be done using Netlify's Split Testing feature, which allows you to deploy multiple versions of your site and serve them on dedicated URLs. To create an opt-in page, you need to build and deploy a branch with the new feature, then use Netlify's Split Testing feature to allocate traffic between the production branch and the beta branch. You can use a cookie in the browser to determine which branch to serve, and provide a UI for users to opt in or out of the beta by setting the cookie value or clearing it. To prevent direct access to the beta branch, you can add redirect rules using Netlify's Redirects API. This approach allows you to generate each variant of your site at build time, serving static assets directly from a CDN, and eliminates the need for server-side logic or infrastructure management.