update_term_meta not working

A terms slug is not stored as meta/custom fields, and it’s definitely not post meta, so update_term_meta and update_post_meta are innapropriate and can never be used to achieve your goal.

The correct function is wp_update_term, however you’ve used that incorrectly too:

• $term->id isn’t a thing, if we look at the WP_Term class and the examples on the official docs, we see $term->term_id is what should be used
• there is no code checking the return value of wp_update_term, if it failed there would have been a WP_Error object with an error message explaining what happened.
• error_log($term->slug); doesn’t fetch the new slug from the database or make a new query to check for a new slug, this cannot be used to test if it worked. You need to check the return value of wp_update_term instead

From the official dev docs:

Return

(array|WP_Error) An array containing the term_id and term_taxonomy_id, WP_Error otherwise.

More examples and information at:

• https://developer.wordpress.org/reference/functions/wp_update_term/
• https://developer.wordpress.org/reference/classes/wp_term/