Modified date of Records in Data extension using SSJS

It seems you won't be able to retroactively extract this data even using WSProxy.

Let's assume you have a data extension called WSProxyDE with two columns:

• TextID - your primary key
• NumberValue - a numeric value you update

Insert data into this DE and modify one row (let's say the last of 3 rows).

Run the script on CloudPage:

<script runat="server">
Platform.Load("core","1.1.5");

var deName = "WSProxyDE"; //The name of your DE
var cols = ["TextId", "NumberValue"]; // columns you want to retrieve

var proxy = new Script.Util.WSProxy();
var object = "DataExtensionObject[dekey]".replace("dekey", deName);
var data = proxy.retrieve(object, cols);

Write(Stringify(data.Results));
</script>

You will get a response in the form of a stringifed array, but if you extract the third object representing your updated row, you will get this:

{
       "Name":null,
       "Keys":null,
       "Type":"DataExtensionObject",
       "Properties":[
          {
             "Name":"TextId",
             "Value":"ThirdContact"
          },
          {
             "Name":"NumberValue",
             "Value":"3"
          }
       ],
       "Client":null,
       "PartnerKey":null,
       "PartnerProperties":null,
       "CreatedDate":"0001-01-01T00:00:00.000",
       "ModifiedDate":null,
       "ID":0,
       "ObjectID":null,
       "CustomerKey":null,
       "Owner":null,
       "CorrelationID":null,
       "ObjectState":null,
       "IsPlatformObject":false
    }

The value you're seeking - ModifiedDate - is null (and the CreatedDateValue is also useless).

To achieve this you would need a column where you could store the date when a record was modified, but I see no way of extracting this data for that's already in your DE and was modified before you introduce a new column storing the modification date.