This problem has been fixed in Quartus® Prime software version 16.1.2. All supported values below 60 ohms can be selected for 1.2-V POD I/O standard targeting DDR4 and QDRIV protocols.
For the memory interface protocols which doesn’t use 1.2-V POD I/O standard (e.g. DDR3, RLDRAM3), input termination values below 50 ohms have been removed starting with Quartus Prime software version 16.1.1. More details can be found in this KDB answer.