SQL Server 2017

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DECLARE @json   NVARCHAR(MAX) = 
'{
    "productId": "508634800",
    "description": "Generic description",
    "itemInfo": [
        {
            "sku": "4645309",
            "attributes": [
                {
                    "attrName": "Size",
                    "attrValue": "M"
                },
                {
                    "attrName": "Color",
                    "attrValue": "Burgundy"
                }
            ]
        },
        {
            "sku": "4645318",
            "attributes": [
                {
                    "attrName": "Size",
                    "attrValue": "XL"
                },
                {
                    "attrName": "Color",
                    "attrValue": "Burgundy"
                }
            ]
        },
            {
            "sku": "4645400",
            "attributes": [
                {
                    "attrName": "Size",
                    "attrValue": "L"
                },
                {
                    "attrName": "Color",
                    "attrValue": "White"
                }
            ]
        }
    ]
}'

SELECT /*sku,*/ Size, Color
FROM 
(
SELECT productId, sku, attrName, attrValue
FROM
	OPENJSON(@json)
WITH ( productId  int 'strict $.productId',  
description  NVARCHAR(MAX) '$.description',
		itemInfos	NVARCHAR(MAX) '$.itemInfo' AS JSON
	) 
	 outer apply openjson( itemInfos ) 
                     with ( sku nvarchar(8) '$.sku' ,
							attributes	NVARCHAR(MAX) '$.attributes' AS JSON	)
					outer apply openjson( attributes ) 
						 with ( attrName nvarchar(MAX) '$.attrName' ,
								attrValue	NVARCHAR(MAX) '$.attrValue'	)
) AS SUBJSONQUERY
PIVOT(
	MAX(SUBJSONQUERY.attrValue) FOR SUBJSONQUERY.attrName IN (Size,Color) ) AS pvtSubJsonQuery

Size	Color
M	Burgundy
XL	Burgundy
L	White