根据删除文档Firebase文档
警告：删除单据并不删除其子集合！
然后呢

不建议客户端删除收藏。

相反，您应该删除具有Delete data with a Callable Cloud Function子集合的文档，并从客户端应用程序调用它。
Firebase Docs还提供了一个示例firebase函数来删除文档，您可以在这里检查：解决方案：使用可调用的云函数删除数据

看来有两个问题需要你去解决。

• 第一个，正如Rohit Kharche之前指出的，删除用户文档不会自动删除其子集合。因此，您需要遍历每个子集合的文档以删除它们。
• 第二个问题与Firebase批处理有关，Firebase批处理限制为500个文档。如果子集合中的文档超过500个，则批处理操作将失败，删除过程将无法完成。

下面的代码解决方案解决了所有这些问题：

void deleteUser() async {
  final sp = context.watch<SignInProvider>();

  /* Since we are using batch to delete the documents usually the max batch size
  * is 500 documents.
  */
  const int maxBatchSize = 500;

  // Define the users collections;
  final List<String> collections = [
    "books",
    "pdfs",
  ];

  try {
    // Get the current user's ID
    final String? uid = FirebaseAuth.instance.currentUser?.uid;

    //loop through collection and delete each collection item from a user
    for (String collection in collections) {
      // Get the snapshot of a collection.
      final QuerySnapshot snapshot = await FirebaseFirestore.instance
          .collection('Users')
          .doc(uid)
          .collection(collection)
          .get();
      // Get the list of documents;
      final List<DocumentSnapshot> querySnapshotList = snapshot.docs;

      // Get the length of the list(no of documents in the collections)
      final int documentLengths = querySnapshotList.length;

      /* The below code allows us too loop through the documents and delete
      them after every 500 documents, since batch is full at 500.
       */
      final int loopLength = (documentLengths / maxBatchSize).ceil();
      for (int _ = 0; _ < loopLength; _++) {
        final int endRange = (querySnapshotList.length >= maxBatchSize)
            ? maxBatchSize
            : querySnapshotList.length;
        // Define the batch instance
        final WriteBatch batch = FirebaseFirestore.instance.batch();
        for (DocumentSnapshot doc in querySnapshotList.getRange(0, endRange)) {
          batch.delete(doc.reference);
        }
        // Commit the delete batch
        batch.commit();
        querySnapshotList.removeRange(0, endRange);
      }
    }
    // Delete the user profile
    await FirebaseFirestore.instance.collection('Users').doc(uid).delete();

    // Log out user from all platforms.
    sp.userSignOut();
    /*
    // Log out user from all platforms.
    Future.wait([
      _googleSignIn.signOut(),
      _facebookAuth.logOut(),
      _firebaseAuth.signOut(),
    ]);
     */
  } catch (e) {
    // Handle any errors that occur during the deletion process
    print('Error deleting user document: $e');
    // Display an error message to the user
    ScaffoldMessenger.of(context).showSnackBar(
      SnackBar(content: Text('Failed to delete account')),
    );
  }
}