Is it a condition for the existence of li'an that there be the hadd punishment for slander and zina? And how is paternity denied in the absence of the application of these hudud punishments in many Muslim countries?

It is obligatory upon the Muslim rulers to implement all the prescribed Islamic punishments (hudud). The non-implementation of some of them does not preclude the implementation of others. Li'an (oath of condemnation) is not limited to an accusation of adultery; it is also required for denying paternity, even if the husband does not accuse his wife of adultery. Ibn Rushd mentioned that the types of claims necessitating li'an are: an accusation of adultery and the denial of pregnancy.