My understanding is that humidity was kept low on aircraft due to corrosion issues. So not sure why they were an option on some aircraft. But it seems that in nearly all instances e airlines deactivated them due to corrosion and/or wear issues, especially for avionics in the cockpit.
I know a 787 operator that is very unhappy with the extra humidity in the cockpit, because even though the aircraft structure (being cfrp) can easily handle the extra humidity (from 5% on standard aluminum frames to 15% on 787) there are many other components on the aircraft that are suffering.
The 350 comes with an optional cockpit humidifier, however, I have yet to find a picture of a 350 cockpit from any airline where that option is installed.