On just about all airplanes the Stall Protection System disengages the autopilot automatically whenever a range of parameters is exceeded. One of them is approaching stall. Generally the Autopilot will be kicked off automatically just before stick shaker onset on a traditional non-FBW airplane, and it can't be reengaged until the parameters that were ...
In the AF447 accident, the autopilot disconnected because of inconsistent airspeed:
Temporary inconsistency between the airspeed measurements, likely following
the obstruction of the Pitot probes by ice crystals that, in particular, caused the
autopilot disconnection and the reconfiguration to alternate law;
(BEA - Final Report)
As long as the airspeed ...