LWRP FullScreenQuad feature example. This can be used to extend LWRP render to render a full screen quad.

FullScreenQuad.cs

namespace UnityEngine.Rendering.LWRP
{
    // A renderer feature contains data and logic to enqueue one or more render passes in the LWRP renderer.
    // In order to add a render feature to a LWRP renderer, click on the renderer asset and then on the + icon in 
    // the renderer features list. LWRP uses reflection to list all renderer features in the project as available to be 
    // added as renderer features.
    public class FullScreenQuad : ScriptableRendererFeature
    {
        [System.Serializable]
        public struct FullScreenQuadSettings
        {
            // The render pass event to inject this render feature.
            public RenderPassEvent renderPassEvent;
            
            // Material to render the quad. 
            public Material material;
        }

        // Contains settings for the render pass.
        public FullScreenQuadSettings m_Settings;
        
        // The actual render pass we are injecting.
        FullScreenQuadPass m_RenderQuadPass;

        public override void Create()
        {
            // Caches the render pass. Create method is called when the renderer instance is being constructed. 
            m_RenderQuadPass = new FullScreenQuadPass(m_Settings);
        }

        public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData)
        {
            // Enqueues the render pass for execution. Here you can inject one or more render passes in the renderer
            // AddRenderPasses is called everyframe. 
            if (m_Settings.material != null)
                renderer.EnqueuePass(m_RenderQuadPass);
        }
    }
}

FullScreenQuadPass.cs

namespace UnityEngine.Rendering.LWRP
{
    // The render pass contains logic to configure render target and perform drawing.
    // It contains a renderPassEvent that tells the pipeline where to inject the custom render pass. 
    // The execute method contains the rendering logic.
    public class FullScreenQuadPass : ScriptableRenderPass
    {
        string m_ProfilerTag = "DrawFullScreenPass";

        FullScreenQuad.FullScreenQuadSettings m_Settings;

        public FullScreenQuadPass(FullScreenQuad.FullScreenQuadSettings settings)
        {
            renderPassEvent = settings.renderPassEvent;
            m_Settings = settings;
        }

        public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData)
        {
            Camera camera = renderingData.cameraData.camera;

            var cmd = CommandBufferPool.Get(m_ProfilerTag);
            cmd.SetViewProjectionMatrices(Matrix4x4.identity, Matrix4x4.identity);
            cmd.DrawMesh(RenderingUtils.fullscreenMesh, Matrix4x4.identity, m_Settings.material);
            cmd.SetViewProjectionMatrices(camera.worldToCameraMatrix, camera.projectionMatrix);
            context.ExecuteCommandBuffer(cmd);
            CommandBufferPool.Release(cmd);
        }
    }
}

Try adding:

context.Submit();

Thank you! That was the missing bit :)

What if I want the Projection matrix to be set to the actual rendering camera's projection matrix? So that the Material's shader can correctly compute things like view direction? What should the matrix in cmd.DrawMesh be then to make sure the quad is still "in front of the camera" ?

Edit: I did it as follows, I did not call cmd.SetViewProjectionMatrices, but instead I did

// Create a transformation that places the Quad 1 unit in front of the camera.
var fullScreenQuadMatrix = Matrix4x4.TRS(camera.transform.TransformPoint(Vector3.forward), camTransform.rotation, Vector3.one);
cmd.DrawMesh(RenderingUtils.fullscreenMesh, fullScreenQuadMatrix, _material, 0, _materialPassIndex);

The RenderFeature settings are not displayed in the ReorderableList when added to a ForwardRenderer.

So apparently, m_ in front of the word settings stops the property from displaying. smh