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: