Which 3GPP release supports 256 QAM in the downlink?

One of the LTE features introduced in Rel-12 is the support for higher order modulation, that is 256QAM for DL data transmission to enhance the spectral efficiency and the system throughput.

From Rel 12 , 3GPP defines (support) 256 QAM in Downlink.

Just what is QAM-256? It refers to Quadrature Amplitude Modulation , which is the means by which a carrier signal, such as an LTE waveform, transmits data and information. Ideally, a waveform/symbol carries as much data as possible in order to achieve higher data rates and increase spectral efficiency.

The required bandwidth is related to bit rate and the modulation order M. It is so that the double sided bandwidth w = symbol rate= bit rate rb/ divided by the number of bit per symbol n . The number of bits per symbol is = log 2M with M is the M is the QAM modulation order.