wp-includes/l10n/class-wp-translation-file.php:149Returns all headers.
array<string, public function headers(): array { if ( ! $this->parsed ) { $this->parse_file(); } return $this->headers; }Introduced in 6.5.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/l10n/class-wp-translation-file.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.