Inspecting WP_Rest_Request

As noted above in Sally CJ’s comment, and as seen in the Wp Codebase, those are protected properties, so they are meant to be accessed only through the multiple get and set methods that the class exposes:

/**
  * Sets the header on request.
  *
  * @since 4.4.0
  *
  * @param string $key   Header name.
  * @param string $value Header value, or list of values.
  */
public function set_header( $key, $value ) {
    $key   = $this->canonicalize_header_name( $key );
    $value = (array) $value;

    $this->headers[ $key ] = $value;
}

// etc...

If you needed to access them directly, you would extend the class:

class MY_WP_REST_Request extends WP_REST_Request {

    public $params;
    // etc...
}

Though in my case, using the access methods provided is proving to be fine.