This Unity editor script exports individual sub-sprites from a selected sprite asset as PNG files.

ExportSubSprites.cs

using System.IO;
using UnityEditor;
using UnityEngine;

namespace Long18.Utils
{
    /// <summary>
    /// Utility class for exporting individual sub-sprites from a selected sprite asset to PNG files.
    /// </summary>
    public class ExportSubSprites : Editor
    {
        [MenuItem("Assets/Export Sub-Sprites")]
        public static void ExportSelectedSubSprites()
        {
            string folderPath = EditorUtility.OpenFolderPanel("Export sub-sprites to folder", "", "");
            if (string.IsNullOrEmpty(folderPath)) return;

            foreach (var selectedObject in Selection.objects)
            {
                if (selectedObject is Sprite sprite)
                {
                    Texture2D extractedTexture = SpriteExtractor.Extract(sprite);
                    SpriteSaver.Save(extractedTexture, folderPath);
                }
            }

            SpriteSaver.OpenExportFolder(folderPath);
        }

        [MenuItem("Assets/Export Sub-Sprites", true)]
        private static bool CanExportSelectedSubSprites()
        {
            return Selection.activeObject is Sprite;
        }
    }

    /// <summary>
    /// Provides functionality to extract the texture region of a sprite.
    /// </summary>
    public static class SpriteExtractor
    {
        /// <summary>
        /// Extracts the texture region occupied by the sprite and returns a new Texture2D.
        /// </summary>
        /// <param name="sprite">The sprite to extract from.</param>
        /// <returns>A new Texture2D containing the extracted sprite region.</returns>
        public static Texture2D Extract(Sprite sprite)
        {
            Texture2D sourceTexture = sprite.texture;
            Rect spriteRect = sprite.textureRect;

            Texture2D readableTexture = CreateReadableTexture(sourceTexture);
            Texture2D croppedTexture = CropTextureToSprite(readableTexture, spriteRect);

            croppedTexture.name = $"{sprite.name}";

            return croppedTexture;
        }

        /// <summary>
        /// Creates a readable copy of the source texture by rendering it into a temporary RenderTexture.
        /// </summary>
        private static Texture2D CreateReadableTexture(Texture2D sourceTexture)
        {
            RenderTexture temporaryRenderTexture =
                RenderTexture.GetTemporary(sourceTexture.width, sourceTexture.height, 0);
            Graphics.Blit(sourceTexture, temporaryRenderTexture);

            RenderTexture previousRenderTexture = RenderTexture.active;
            RenderTexture.active = temporaryRenderTexture;

            Texture2D readableTexture = new Texture2D(sourceTexture.width, sourceTexture.height);
            readableTexture.ReadPixels(new Rect(0, 0, temporaryRenderTexture.width, temporaryRenderTexture.height), 0,
                0);
            readableTexture.Apply();

            RenderTexture.active = previousRenderTexture;
            RenderTexture.ReleaseTemporary(temporaryRenderTexture);

            return readableTexture;
        }

        /// <summary>
        /// Crops the given readable texture to match the sprite's defined rectangle.
        /// </summary>
        private static Texture2D CropTextureToSprite(Texture2D readableTexture, Rect spriteRect)
        {
            Texture2D croppedTexture = new Texture2D((int)spriteRect.width, (int)spriteRect.height);
            Color[] spritePixels = readableTexture.GetPixels((int)spriteRect.x, (int)spriteRect.y,
                (int)spriteRect.width, (int)spriteRect.height);

            croppedTexture.SetPixels(spritePixels);
            croppedTexture.Apply();

            return croppedTexture;
        }
    }

    /// <summary>
    /// Handles saving Texture2D objects to PNG files.
    /// </summary>
    public static class SpriteSaver
    {
        /// <summary>
        /// Saves a Texture2D as a PNG file to the specified directory.
        /// </summary>
        /// <param name="texture">The texture to save.</param>
        /// <param name="directoryPath">The directory to save the file in.</param>
        public static void Save(Texture2D texture, string directoryPath)
        {
            if (!Directory.Exists(directoryPath))
            {
                Directory.CreateDirectory(directoryPath);
            }

            string filePath = Path.Combine(directoryPath, $"{texture.name}.png");
            File.WriteAllBytes(filePath, texture.EncodeToPNG());
        }

        /// <summary>
        /// Opens the specified directory in the system's file explorer.
        /// </summary>
        /// <param name="directoryPath">The directory to open.</param>
        public static void OpenExportFolder(string directoryPath)
        {
            if (Directory.Exists(directoryPath))
            {
                EditorUtility.RevealInFinder(directoryPath);
            }
        }
    }
}