Adding a domain to a project will now also suggest adding its www counterpart. This ensures visitors can always access your site, regardless of whether they type www when entering the domain, or not.
Using a www domain guarantees that the Vercel Edge Network can reliably and securely route incoming traffic as quickly as possible, so redirecting non-www to the www domain is recommended. Redirecting the other way works too if you prefer a cleaner URL address.
Existing domains are not affected by this change, but we recommend ensuring that your project already has a www redirect in place.
Check out the documentation as well.