CNAME Flattening
CNAME Flattening allows a CNAME record to be created for the root domain without violating DNS specifications.
CloudflareはCNAME flattingをサポートしている
Unless your DNS provider supports CNAME flattening for apex domains, we strongly recommend setting the www subdomain (or another subdomain) as your primary domain. Our blog post How to Set Up Netlify DNS has more details on why we recommend that configuration.
https://docs.netlify.com/domains-https/custom-domains/multiple-domains/#apex-domains-and-www-subdomains