wp-includes/category-template.php:1251$tagintoptional0stringfunction tag_description( $tag = 0 ) { return term_description( $tag );}Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/category-template.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.