My 875 (the first WH hydro) has the disengagement lever. There are no safety switches, so, IMO, the only reason to have the lever would be to make engine starting easier.
All my C series hydros have the lever. My '88 520 has the lever. My '95 312 does not have the lever. It appears the lever was eliminated about 1990.
If this is the case, I am guessing , the elimination of the levers in the later years was strictly for a cost savings.
If we give Garry a couple minutes research time, he will probably be able to confirm when and why this option was discontinued.