A low compression, mogas certified version of the IO-390 was being developed specifically for the Panthera. Lycoming decided not to produce this engine after testing due to a substantial decrease in power output that resulted from the lower than expected compression ratio required to meet certified engine detonation margins. Lycoming felt that a parallel valve engine could provide almost the same power running Mogas as the reduced compression IO-390 for less money and weight.
If anything Lycoming may stop producing new angle valve IO-360 variants in favor of the IO-390. Have you noticed that a new IO-390 costs less than a new angle valve IO-360? Hmm......