If you try to remove a class with a nonexistent datatype, eZ will crash with a fatal error.
This problably happens because "* ezpublish_legacy/kernel/classes/ezcontentclass.php#815*", removableInformation function
It shouldn't be an impediment to remove a class with a nonexistent datatype.
1. Create a custom datatype. A simple way to do this is to install eZComments package. eZComments will change the article class and add its own custom datatype.
2. Duplicate article class and name it "article_1".
3. Remove the "article_1" class and confirm there will be no problem.
4. Delete the ezcomments extension
5. Try to remove article class and you will see a fatal error since the the comments datatype does not exist.