wp-admin/includes/class-wp-filesystem-ftpext.php:561Checks if a file or directory is writable.
$pathstringbool public function is_writable( $path ) { return true; }Introduced in 2.5.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-admin/includes/class-wp-filesystem-ftpext.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.