Visually indicate that a Unity inspector value is required.

RequiredFieldAttribute.cs

using System;
using UnityEngine;

/// <summary>
/// Indicates that an inspector field must have its value assigned during authoring.
/// </summary>
[AttributeUsage(AttributeTargets.Field)]
public class RequiredFieldAttribute : PropertyAttribute
{
}

RequiredFieldPropertyDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(RequiredFieldAttribute))]
public class RequiredFieldPropertyDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        SerializedProperty prop = property.serializedObject.FindProperty(property.propertyPath);
        bool isNotNull = CheckNotNull(prop);
        
        if (!isNotNull)
        {
            var color = GUI.color;

            var lineRect = new Rect(position)
            {
                xMax = position.xMin,
                xMin = position.xMin - 3,
                yMin = position.yMin + 1,
                yMax = position.yMax - 1
            };
            EditorGUI.DrawRect(lineRect, new Color(0.75f, 0f, 0f));
            
            GUI.color = color;
        }
        
        EditorGUI.PropertyField(position, property, label, true);
    }

    private static bool CheckNotNull(SerializedProperty property)
    {
        switch (property.propertyType)
        {
            case SerializedPropertyType.String:
                return !string.IsNullOrEmpty(property.stringValue);
            case SerializedPropertyType.ObjectReference:
                return property.objectReferenceValue != null;
            case SerializedPropertyType.AnimationCurve:
                return property.animationCurveValue.keys.Length > 0;
            case SerializedPropertyType.ExposedReference:
                return property.exposedReferenceValue != null;
            default:
                return true;
        }
    }
}