Opinions differ on this subject. It is often assumed that screed that was installed years ago will have dried out over time. This can certainly be the case. However, to receive the full manufacturer’s warranty for your laminate floor, you should exclude all possible sources of damage. So when laying laminate, also think about a vapour barrier film – that way you are on the safe side and won't be annoyed later about ripples in the laminate flooring due to moisture or about the warranty being voided.