I'm not sure what you mean, as Gitea has SSO using many different ways, such as LDAP, OAuth2/OIDC, OpenID, SMTP, and more.
> what happens when a community member wants to implement SAML for the community edition
It's surely just business model, but I was intrigued and thought that maybe there were some kind of incompatible licensing in popular libraries people use for these so-called "premium features"