Traditionally this came with the requirement to use Cloudflare services, at least the free tier to manage the domain. In other words the domain was delegating to Cloudflare’s name servers and this was not negotiable. Today this is no longer the case. Shortly after launching registrar services to the public they added an API endpoint /registrar that can change a domain’s delegation to NS outside of Cloudflare. A proof of concept and example script can be found on GitHub.