set-delay-promotion delays promotion for generation gen-num, which means that objects are promoted to the next generation in the second garbage collection (GC) that they survive in generation gen-num. By default, objects are promoted in the first GC.
set-delay-promotion is implemented only in 64-bit LispWorks. It does nothing in the Mobile GC and its return value is not meaningful. It is not relevant to the Memory Management API in 32-bit implementations.
LispWorks User Guide and Reference Manual - 20 Sep 2017