Tested configurations:

• Copter - 4.5.7 - CubeOrangePlus
• Copter - 4.4.4 - CubeOrangePlus

Plane-"Advanced Failsafe features" checkbox does not affect the compilation.
The "AFS_" parameters won't be present on the autopilot regardless of the checkbox state.

I thought this might be a web GUI issue, but I tried compiling the AP manually with the AFS enable definition passed to the waf configure command line, but it didn't work either.
Moreover, the online build server compilation log seems like this feature is being included and compiled.

Found fix: Manually editing the config.h file and enabling AFS define there.