The distributed memory access toggle in Cycles preferences would show up when a user has two GPUs that can access each other's memory, but only one of them is supported by Cycles. For example the AMD RX 5700XT and AMD Vega 64 can access each other's memory, but only the 5700XT is supported by Cycles. Pull Request: https://projects.blender.org/blender/blender/pulls/140521