DAO has no use for a decocker and SA only should not be carried hammer
down on a loaded chamber in the first place. We have to be talking
about DA/SA.
With modern single action, should be able to carry hammer down on a loaded
chamber. The trigger lifts a striker in to position between the hammer and
firing pin has been required for at least 20 years. So a dropped firearm
would not discharge.
There is just little advantage since cocking most of them is somewhat
cumbersome on the draw stroke. On a 1911, that is not even a
recognized "condition".