Starting April 14, 2020, sites without a custom domain are being moved from site-name.netlify.com to site-name.netlify.app . Traffic to any site-name.netlify.com address will be forwarded to site-name.netlify.app using an HTTP 301 redirect. New websites will also have URLs ending with netlify.app by default.

This could cause issues for sites which don't have CORS configured properly for the netlify.app domain or where proxies (such as a Cloudfront origin or Cloudflare) are being used in front of the Netlify server URL, as proxies often don't follow 301 redirects.

Netlify sent an email about this dated April 1, 2020 but I just noticed this email by chance due to a Google reminder today on my phone. Wonder how many people will be caught off-guard by this tomorrow, especially due to the long Easter weekend just before this deadline.