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
353789c559