a military derivative of a civilian type and the stall warning system was deactivated in service
I flew a test program on a Siai Marchetti 1019, which is a derivative of a Cessna 305/L19/O-1 Bird Dog. The 305 does have a stall warning system, the 1019 does not. The 305 had a very benign, Cessna like stall, which was well warned aerodynamically.
Certification design requirements allow that the warning of the stall may be purely aerodynamic, without a supplementary system, of the plane is approved that way. For the 305, and indeed, the 185, the TCDS specifies a functioning stall warning system. It's all a matter of how it was certified, but if it was certified with it as specified equipment, it's gotta work!