The GC combinations ParNew + SerialOld and DefNew + CMS are very rarely used. There are no use cases that we know of where these are superior to other GC combinations.
We are currently not doing active testing on these combinations.
To simplify the GC combination matrix and make sure that we have resources to test all GC combinations that we support we would like to remove these two combinations. The plan is to deprecate these combinations in JDK8 and remove them in JDK9.