According to multiple suppliers, fast-speed 18W (9V/2A) charging may not be available for third party USB-C adapters that don't support the USB Power Delivery Revision 3.0 specification certification.
As for USB PD 3.0 standard, in addition to that EMIC low voltage power was changed from more than 5 V to more than 3.0V, "C-AUTH"(USB Type-C Authentication) was added in the equipment and cable connected to distinguish regular item. The next iPhone might be applicable to "C-AUTH". Those who are working on it say that if unauthorized item is used, USB PD 3.0 standard policy that does not supply the power exceeding "2.5W (5V/0.5A)" is applied.
Earlier this month, another report from the same site claimed that third party manufacturers will purportedly gain MFi access to using an improved C48B Lightning chip in November or December of 2018. They are expected to release cables with the new chip in 2019.
Check out some purported photos of Apple's new 18W charger here. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.