<?php
$products = [[
        "id" => 1947
        ,"cat_id" => 48
        ,"tax_class_id" => 18
        ,"sku" => "620068-429-S"
        ,"qty_ordered" => 5
    ],[


        "id" => 1947
        ,"cat_id" => 48
        ,"tax_class_id" => 28
        ,"sku" => "620068-429-M"
        ,"qty_ordered" => 9
    ],[


        "id" => 1947
        ,"cat_id" => 48
        ,"tax_class_id" => 18
        ,"sku" => "620068-429-L"
        ,"qty_ordered" => 9
    ],[


        "id" => 1947
        ,"cat_id" => 48
        ,"tax_class_id" => 28
        ,"sku" => "620068-429-XL"
        ,"qty_ordered" => 9
    ]];
    
     $val = [];
    $newArray = [];
    foreach ($products as $product) {

      $key = array_search($product['tax_class_id'],$val);
      if(!$key) {
        $val[] = $product['tax_class_id'];
        $key = array_search($product['tax_class_id'],$val);
      }
      $newArray[$key][] = $product;
    }
    var_dump($newArray);