Thanks for the replies. It sounded strange to me that a keel would fall off but I wanted to be sure just in case.
This occurrence would be extremely rare, more likely due to lack of maintenance or over forcing it. At the top are 2 nuts on the keel screw shaft tightened together, with the lower nut having a locking pin, both above the mounting plate.
I head stories of some abuse of the keels hitting rocks and bending the screw but none of these ever detached.