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Try this solution:

$collection = $subject->getCollection();
  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock desc']);

Try this solution:

  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock desc']);

Try this:

$collection = $subject->getCollection();
  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock desc']);

Try this solution:

$collection = $subject->getCollection();
  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock desc']);

Try this:

$collection = $subject->getCollection();
  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock ASC']);

Try this:

$collection = $subject->getCollection();
  $collection->getSelect()->joinLeft(
         ['_inventory_table' => 'cataloginventory_stock_item'], 
         "_inventory_table.product_id = e.entity_id", ['is_in_stock']
  );
  $collection->getSelect()->order(['is_in_stock desc']);