Entity extractors and thesaurus aren't preserved after upgrade to SharePoint Server 2016
In SharePoint Server 2013, you deploy custom entity extractors, a custom thesaurus, or both. Then, you upgrade to SharePoint Server 2016. However, after the upgrade, you find that the custom entity extractors, the custom thesaurus, or both, are not available.
This is a known issue.
To resolve this issue, deploy the files for the custom entity extractors or the thesaurus after you upgrade. For more information, see the following Microsoft websites:
Create and deploy custom entity extractors in SharePoint Server 2013
Create and deploy a thesaurus in SharePoint Server 2013
Article ID: 3160712 - Last Review: 08/10/2016 16:36:00 - Revision: 3.0
Microsoft SharePoint Server 2016