The reference processor uses the sentinelRef to mark the end of the lists of discovered References. All the collectors rely on the sentinelRef being allocated in the permgen, hence not moving. So, when permgen is removed this will not work.
We could probably make changes to the collectors to update all pointers to the sentinelRef, but the proposed solution is to remove the sentinelRef, and instead let the last Reference in a discovered chain point back to itself. Just like we do with the next field.