Unity CommandBuffer replacement for GrabPass - works with multiple separate cameras.

CameraTrackingRefraction.cs

using UnityEngine;
using UnityEngine.Rendering;

// This script is added to cameras automatically at runtime by the ObjectNeedingRefraction scripts.
public class CameraTrackingRefraction : MonoBehaviour {

	[System.NonSerialized]
	public int lastRenderedFrame = -1;

	Camera cam;
	CommandBuffer buffer;

	void Awake () {
		cam = Camera.current;
		cam.depthTextureMode = DepthTextureMode.Depth;
	}

	public void SetIsNeededThisFrame () {
		lastRenderedFrame = Time.frameCount;
		if (buffer == null)
			CreateCommandBuffer ();
		enabled = true;
	}

	void Update () {
		if (lastRenderedFrame < Time.frameCount - 1) {
			DestroyCommandBuffer ();
			enabled = false;
		}
	}

	void DestroyCommandBuffer () {
		cam.RemoveCommandBuffer (CameraEvent.AfterSkybox, buffer);
		buffer = null;
	}

	void CreateCommandBuffer ()
	{
		buffer = new CommandBuffer();
		buffer.name = "Grab screen";

		int screenCopyID = Shader.PropertyToID("_RefractionGrabTexture");
		buffer.GetTemporaryRT (screenCopyID, -1, -1, 0, FilterMode.Bilinear);
		buffer.Blit (BuiltinRenderTextureType.CurrentActive, screenCopyID);

		buffer.SetGlobalTexture("_RefractionGrabTexture", screenCopyID);

		cam.AddCommandBuffer (CameraEvent.AfterSkybox, buffer);
	}
}

ObjectNeedingRefraction.cs

using System.Collections.Generic;
using UnityEngine;

// Place this script on all GameObject with renderers with materials
// that use shaders that need the _RefractionGrabTexture "grab texture".
// The GrabPass in the shader can then be removed and the rest should ideally work as-is.
public class ObjectNeedingRefraction : MonoBehaviour {

	static Dictionary<Camera, CameraTrackingRefraction> m_Trackers =
		new Dictionary<Camera, CameraTrackingRefraction>();

	public void OnWillRenderObject () {
		var cam = Camera.current;
		if (!cam)
			return;

		CameraTrackingRefraction tracker;
		if (!m_Trackers.TryGetValue (cam, out tracker)) {
			tracker = cam.GetComponent<CameraTrackingRefraction> ();
			if (!tracker)
				tracker = cam.gameObject.AddComponent<CameraTrackingRefraction> ();
			m_Trackers[cam] = tracker;
		}

		tracker.SetIsNeededThisFrame ();
	}
}