According to Unity's documentation, the term "random write" denotes if shaders can write into arbitrary locations of a textures. (https://docs.unity3d.com/ScriptReference/RenderTexture-enableRandomWrite.html).
When using D3D11, SupportsRandomWriteOnRenderTextureFormat() should return whether a texture format can be used as a Unordered Access View (UAV). UAVs support *simultaneous* random reads and writes.
In Metal, the term UAV is not used. We need to check the Metal device readWriteTextureSupport. The official Metal documentation of both MTLReadWriteTextureTier.tier1 and MTLReadWriteTextureTier.tier2 does not mention any rg16 format. Therefore the behavior of Unity 6 that returns false for RG16 is correct and this will be fixed on 2022.3.