It is the last version to support AVR-based radios (9X/9XR with stock boards). Stability:
OpenTX is a popular open-source firmware for radio control systems, widely used in the RC community. The latest version, OpenTX 2.1.9, offers a range of exciting features, improvements, and bug fixes. Here's where you can download it and what you need to know. opentx 2.1.9 download