@Jiawei-Shao brought this up in Dawn quite a while ago, but I'm not sure whether we ever made an explicit decision about it in WebGPU. Messages from Jiawei:

Actually on Metal "Function Buffer Read-Writes" requires iOS_GPUFamily3_v2 (A9+, iOS10+) and OSX_GPUFamily1_v2 (macOS 10.12+), so it seems not all Metal devices support storage buffer in fragment shader.
(https://developer.apple.com/library/archive/documentation/Miscellaneous/Conceptual/MetalProgrammingGuide/WhatsNewiniOS10tvOS10andOSX1012/WhatsNewiniOS10tvOS10andOSX1012.html#//apple_ref/doc/uid/TP40014221-CH14-SW1)

Also I see in MoltenVK "fragmentStoresAndAtomics" is only enabled on macOS, not iOS: https://github.com/KhronosGroup/MoltenVK/blob/master/MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm#L1052

So that makes it unsupported up through A8, iOS 9, and macOS10.11.

Additionally, fragmentStoresAndAtomics is not supported on some Vulkan hardware. There is very little Android data on gpuinfo.org, but what is there seems to show that it is unsupported up through Adreno 4xx.