Got an answer from the Nintext support:

"...the workflow definitions are stored as xml files in libraries within the site and not in the databases. So, unfortunately there is no possible way to search for workflows and forms which use absolute url links inside the action/control. You will have to check all the workflows/form manually.

For more info on Workflow Storage Locations:
https://community.nintex.com/t5/How-To/Workflow-Storage-Locations/ta-p/86666"

As all definition is stored in the "...hidden library called NintexWorkflows..." we will use a small Powershell script for our research.