wp-signup.php:326Validates user sign-up name and email.
arrayfunction validate_user_form() { return wpmu_validate_user_signup( $_POST['user_name'], $_POST['user_email'] );}Introduced in MU (3.0.0). Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-signup.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.