For regular Azure AD, the new app registration experience is generally available and the legacy experience has been deprecated. https://developer.microsoft.com/en-us/identity/blogs/deprecation-of-app-registrations-legacy-experience-and-the-application-registration-portal-apps-dev-microsoft-com/
For Azure AD B2C, there is a separate experience and you can choose between using the current GA one and the preview one. https://learn.microsoft.com/en-us/azure/active-directory-b2c/tutorial-register-applications?tabs=applications
https://learn.microsoft.com/en-us/azure/active-directory/develop/app-registrations-training-guide-for-app-registrations-legacy-users
So you are correct. In Azure AD it's generally available but in B2C it's in preview. Some of the difference between the two experiences are described in those articles:
In the legacy experience, to register an app you were required to provide: Name, Application type, and Sign-on URL/Redirect URI. The apps that were created were Azure AD only single-tenant applications. They only supported organizational accounts from the directory the app was registered in.
In the new experience, you must provide a Name for the app and choose the Supported account types. You can optionally provide a Redirect URI. If you provide a redirect URI, you'll need to specify whether it's web/public (mobile and desktop).
Hope this helps!