What unexpected data might be stored in terms table and related tables?

There are a lot of data stored in those 4 tables

  • wp_terms stores the term_id, name, slug and term_group ID for a specific term.

  • wp_termmeta stores any meta data which you add to a term. This is a new feature introduced in WordPress 4.4

  • wp_term_taxonomy holds the other info of a term stored in wp_terms. This tables stores the relationship between the term ID and other relative info such as term_taxonomy_id, taxonomy, description, parent and term count

  • wp_term_relationships stores the relationship between posts and their terms. This table just stores the object_id (which is the post ID) and the term_taxonomy_id (which is the term ID) and term_order. Deleting this table will result in posts loosing their term relationships, ie, there will be no more terms associated to posts

It should be relative safe to delete the wp_termmeta table. This should only break functions which relies on this term meta data at worst. Deleting ANY of the other three tables will break terms and any relationships they might have to posts. It is really not a clever idea to delete tables blindingly.

EDIT

This question is asking what non-obvious data may be stored in those tables as well

Apart from wp_termmeta and apart from storing term data and the post ID’s to which those terms are connected to, there should not be any non-relevant, non-obvious data stored in these tables by default. Some plugins might introduce some extra columns to store additional data related to terms, but this is as far as it should go. WorPress itself does not store any additional data from anywhere in these tables

What is obvious though, if you delete these tables, any function/class/code which relies on this data will be broken

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