wp-includes/class-wp-http-encoding.php:196What encoding the content used when it was compressed to send in the headers.
string public static function content_encoding() { return 'deflate'; }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/class-wp-http-encoding.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.