Because this is an older legacy version, you won't find it on the main "Downloads" slider on the OpenTX website. You have to dig into the archives.

(Note: If you have trouble starting the simulator on Windows XP, you may need the Microsoft Visual C++ 2008 Redistributable Companion 2.1.9 dmg : Available in 32-bit DEB 64-bit DEB 32-bit RPM Key Features of the 2.1 Branch Telemetry Overhaul

You need the specific software tool to load the firmware onto your radio.

: 2.1 introduced a major "Telemetry Rewrite." Sensors are no longer hardcoded; you must use the "Discover New Sensors" function on the radio's telemetry page. EEPROM Conversion

Modern OpenTX versions (2.3.0+) include support for FrSky’s ACCESS protocol and R9 2019 hardware. If you are strictly using older ACCST D16 receivers, 2.1.9 provides a no-fuss, glitch-free experience without the need to flash receiver firmware updates.