In JDK-8338977 a set of changes were made to improve the out-of-the-box performance of Parallel GC.

One change that was made, was to never decrease the tenuring threshold. We start with an initial value (InitialTenuringThreshold) of 7 and then have a policy around growing it if needed. The theory is that with parallel we want to avoid Full collections at all cost, so promoting as little as possible should be beneficial.

Promoting less comes with the cost of keeping more objects around longer in the young generation, this can prolong the young gc pause, so we need to investigate that this does not affect the overall performance of parallel.

For non-generational work-loads this will likely not be a good change, but I'm not sure how much parallel needs to care about such use-cases.