Note that the voltage on the external RF module PPM output pin is driven by the battery voltage on the FrSky radios. Especially for SBUS, which is normally 3.3V, this is too high. However, the output is current limited by a 10 kΩ resistor, which limits the current to around 1mA. The input protection diodes of FC processors should be handle the higher voltage at such a low current and connecting the output directly to a flight controller should be relatively safe. But it is still outside the specification and you are doing that on your risk. If you want to be really safe use a voltage divider or a level shifter.