In many CMS like WordPress or Drupal and in the static website we find jQuery and jQuery Migrate ( jquery-migrate.min.js ) scripts which enable some additional functionalities.
jQuery Migrate is made to maintain the depreciated library support which is slowly removed from the main and newer version of the jQuery library.
It is created separately because there are few blogs and websites which still relies on the older version of the jQuery library, and many CMS cannot rely on only new libraries because there would be some users with older versions.
This is the reason why most of the CMS tend to add depreciated features as well.
If you are only rely on the newer version of jQuery, better remove jQuery Migrate.
In static websites it is easy to remove, just navigate to header section and remove the line which loads jquery migrate link or tell developer to do this.
If you are using CMS like WordPress, then you need to dequeue the script.