How to reproduce:
1. Open attached "ReproId" project
2. Build&Run project on Android/iOS and observe the values returned by SystemInfo.graphicsDeviceID and SystemInfo.graphicsDeviceVendorID

Actual result - ID is returned as 0 on OpenGLES2,OpenGLES3, Metal graphics API's, while Vulkan (Android) returns proper device GPU ID's
Expected result - OpenGLES/Metal API's returns proper ID's just like Vulkan (Android) does

Reproducible with - 2018.3.0a11 (trunk), 2018.2.6f1, 2018.1.0a1

Note - https://docs.unity3d.com/ScriptReference/SystemInfo-graphicsDeviceVendorID.html Documentation states that the methods are only implemented on Desktop platforms, if the bug turns out by design, documentation needs an update anyway

DUT:
Reproducible with:
Iphone5s
Samsung Galaxy S8 (SM-G950F), 8.0.0, CPU: Exynos 9 Octa 8895, GPU: Mali-G71
Samsung Galaxy S6 Edge+ (SM-G928F), 7.0, CPU: Exynos 7 Octa 7420, GPU: Mali-T760
Xiaomi MI MAX (MI MAX), 7.0, CPU: Snapdragon 617 MSM8952, GPU: Adreno (TM) 510
Xiaomi Mi 5 (MI 5), 7.0, CPU: Snapdragon 820 MSM8996, GPU: Adreno (TM) 530
Google Pixel (Pixel), 8.1.0, CPU: Snapdragon 821 MSM8996 Pro, GPU: Adreno (TM) 530