Have there been no advances in the determination of effective nuclear charges since Clementi and Raimondi in the 60s?
I would say the most important reason why no development for effective nuclear charges is that the concept of effective nuclear charges has fallen to disuse in the recent years.  I can think of two important reasons: 


Effective nuclear charges is intended to be used to explain electronic structure of isolated atoms.  To this day some atoms can still pose a challenge, but those are the atoms that experience heavy relativistic effects and would need other special treatments than shielding effect along.  However, for the most part, chemists are not interested in atoms anymore.
Its theoretical framework is too simple.  Chemist need their results to be extremely accurate.  A 0.1 eV change can easily be a game changer in an organic reaction.  That, unfortunately, cannot be achieved by a model that has only one adjustable constant. 


However, the idea still lives on.  For example, pseudo-potential or Effective Core Potentials can be considered as more general and modern version of effective nuclear charge concept.  Instead of simply changing the charge but keeping the Coulomb potential unchanged, ECP provides the entire effective potential of core electrons. Valence electrons and sometimes even sub-valence electrons has to be excluded from such treatment to be able to produce results with sufficient accuracy. These pseudo-potentials are actively developed and widely used today, can provide chemically relevant accuracy in complex molecular systems and may qualify as "improvement to effective nuclear charge", although they are evolved into different species already.