Sonoff Ewelink IFTTT no longer free - how to control sonoff devices from smartthigns?

There is a new effort underway to make just that possible, at least for Wi-Fi and other IP protocol home automation. Project CHIP. Amazon, Apple, Google, Samsung, and the zigbee alliance are all involved. But it will probably be two or three years before we actually see interoperable devices.

Matter - smart home connectivity standard (formerly Project CHIP)