A snap to terrain modifier for Mapbox Unity that uses Raycasting.

SnapTerrainRaycastModifier.cs

using Mapbox.Unity.Map;
using Mapbox.Unity.MeshGeneration.Data;
using Mapbox.Unity.MeshGeneration.Modifiers;
using UnityEngine;

[CreateAssetMenu(menuName = "Mapbox/Modifiers/Custom/Snap Terrain Raycast Modifier")]
public class SnapTerrainRaycastModifier : MeshModifier
{
    private const int RAY_LENGTH = 50;

    [SerializeField] private LayerMask terrainMask;

    public override ModifierType Type
    {
        get { return ModifierType.Preprocess; }
    }

    public override void Run(VectorFeatureUnity feature, MeshData md, UnityTile tile = null)
    {
        float worldScale = FindObjectOfType<AbstractMap>().WorldRelativeScale;

        if (md.Vertices.Count > 0)
        {
            for (int i = 0; i < md.Vertices.Count; i++)
            {
                var h = tile.QueryHeightData((float) ((md.Vertices[i].x + tile.Rect.Size.x / 2) / tile.Rect.Size.x),
                    (float) ((md.Vertices[i].z + tile.Rect.Size.y / 2) / tile.Rect.Size.y));

                RaycastHit hit;
                Vector3 rayCenter =
                    new Vector3(md.Vertices[i].x * worldScale + tile.transform.position.x,
                        h * worldScale + RAY_LENGTH / 2,
                        md.Vertices[i].z * worldScale + tile.transform.position.z);

                if (Physics.Raycast(rayCenter, Vector3.down, out hit, RAY_LENGTH, terrainMask))
                {
                    md.Vertices[i] += new Vector3(0, hit.point.y / worldScale, 0);
                }
                else
                {
                    // Raycasting sometimes fails at terrain boundaries, fallback to tile height data.
                    md.Vertices[i] += new Vector3(0, h, 0);
                }
            }
        }
        else
        {
            foreach (var sub in feature.Points)
            {
                for (int i = 0; i < sub.Count; i++)
                {
                    var h = tile.QueryHeightData((float) ((sub[i].x + tile.Rect.Size.x / 2) / tile.Rect.Size.x),
                        (float) ((sub[i].z + tile.Rect.Size.y / 2) / tile.Rect.Size.y));

                    RaycastHit hit;
                    Vector3 rayCenter =
                        new Vector3(sub[i].x * worldScale + tile.transform.position.x,
                            h * worldScale + RAY_LENGTH / 2,
                            sub[i].z * worldScale + tile.transform.position.z);

                    if (Physics.Raycast(rayCenter, Vector3.down, out hit, RAY_LENGTH, terrainMask))
                    {
                        sub[i] += new Vector3(0, hit.point.y / worldScale, 0);
                    }
                    else
                    {
                        // Raycasting sometimes fails at terrain boundaries, fallback to tile height data.
                        sub[i] += new Vector3(0, h, 0);
                    }
                }
            }
        }
    }
}

NOTE: You need to configure your terrain to be added to a layer, and use the layer name with this modifier (default "Terrain").

When making use of snap terrain modifiers such as this, the UnityTile paramter tile is always Null - where is the tile retreived from?