The KB descriptions for the August 2019 CU for SharePoint 2013 included a brief mention that it adds an options to migrate to SharePoint Online into Central Admin. It adds in a green banner in the CA home page and then a new link in the Office 365 area. Both links point to the documentation for downloading the SharePoint Online Migration Tool. Have a look below to view the changes.
https://support.microsoft.com/en-us/help/4475567 - KB description of the change
https://blog.stefan-gossner.com/2019/08/13/august-2019-cu-for-sharepoint-2013-product-family-is-avai... - Stefan Gossner's rollup annoucement post for the August 2019 CU