Rumors suggest only the Apple 18W power adapter and selected third-party adapters will be compatible with the 2018 iPhones.
Mac Otakara claims this, citing information from suppliers.
There are several rumors going that Apple will include an 18W Lightning to USB-C power adapter with the 2018 iPhone models, which enables fast charging. This official Apple adapter won't be for sale the first months after release, and third-party power adapters may require a USB-C Authentication, C-AUTH, to work with the new iPhone models. The C-AUTH is all about extra security, to prevent harmful software or hardware embedded in USB-devices.