Field Description is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 587 of core/lib/Drupal/Core/Entity/ContentEntityBase.php).

I have marked the issue as critical because I see there are two older issues with the similar problem.

https://www.drupal.org/project/taxonomy_import/issues/3238608 →
https://www.drupal.org/project/taxonomy_import/issues/3225261 →

I reworked ImportForm.php to separate loading the CSV or XML from the term operations. There are probably phpcs and documentation issues and it would be best to put the term-related methods in a service. There are also services that should be injected. I also only lightly tested it, so please test it out.

I merged the changes into the 2.0.x-dev branch, please test.

This appears to undo the ability to add custom fields, which was added in ✨ Allow custom field Fixed . The code is moved into ::setAdditionalFields(), which isn't called anywhere.

I thought I already did the part about the CSV and XML readers returning the same array, but I'll look into that more.

Since I made my changes without having used the module with custom fields, what would help is sample CSV and/or XML data (just one term is needed) together with instructions on which field to add, etc.

You are correct. I was looking at the code for version 2.0.12. The commit already done for this issue creates the separate methods.

Attached is a sample import file using a custom field.