Hi Hinmani,
Existing MCS table doesn't have 256 QAM index, It is available till QM6 i.e 28 as MCS index which is 64QAM.
But to aupport 356 QAM there is new IE introduced in RRC capability to know UE support. If UE supported this then eNB uses 256 QAM Mcs table which has MCS index 27 as QM8. older UE uses old table and newer UE uses new Table based on UE capability.