Clipping effect not working on iOS

I'm trying to build a project with this plane clipping effect from Unity 2018.1.2 to iOS 11.4.1 but I don't get the plane clipping effect at all.

Looking at the Xcode console output are the following warnings which I think is why the clipping effect doesn't work:
```
WARNING: Shader Unsupported: 'Custom/StandardClippable' - Pass 'FORWARD' has no vertex shader
WARNING: Shader Unsupported: 'Custom/StandardClippable' - Pass 'FORWARD_DELTA' has no vertex shader
WARNING: Shader Unsupported: 'Custom/StandardClippable' - Pass 'FORWARD' has no vertex shader
WARNING: Shader Unsupported: 'Custom/StandardClippable' - Pass 'FORWARD_DELTA' has no vertex shader
WARNING: Shader Unsupported: 'Custom/StandardClippable' - Pass 'DEFERRED' has no vertex shader
```

Is building to iOS supported at all? If so is there something I should do that I'm missing? Let me know if you need more info. Thanks.

Start by reproducing the iOS 11.4.1 build in Unity 2018.1.2 and inspect the Xcode warnings for Custom/StandardClippable. Compare its FORWARD, FORWARD_DELTA, and DEFERRED passes with the supported iOS shader requirements; done means the plane clipping effect renders without these unsupported-pass warnings.

Written by the indexing model from the issue text.