The new authentication implementation is to provide a single authentication process.
i.e. once the user logs in via Authenticator.Instance.Login/LoginAsync(cred) and its result is asigned to the DefaultAuthenticationContext, all EM instances will be authomatically logged in (unles you explicitly set an EM to not use the DefaultAuthenticationContext).
You asked "How do we do the DefaultAuthenticationContext = null in codes?"...
I'm not sure what you mean. If you want to set DefaultAuthenticationContext = null, you can simply:
Authenticator.Instance.DefaultAuthenticationContext = null;
You will also find this information in the link above.
You also asked "how do we fix issue such as "Implicit login is not allowed when AuthenticationContext is logged out?
If you call logout, you must explicitly log back in in order to perform any operation.