SoE is a feature that only works on our legacy controllers: 800, 2400 for example. In the "old days" when we were doing only FE, we were able to use the remaining 4 pins in the cable to carry serial data as FE used only 4 wires. But With GigE, all 8 pins are used by Ethernet not leaving any for SoE.
Hence, the appearance of Console ports on the newer APs which have GigE ports on them. This has become the defacto for us now.
Hope that helps to understand the feature availability.