'currently still alowed'.....actually on Boeing aeroplanes it's NOT allowed to have any kind of deposits on wing surface,flight controls,leading edge....
Only thing allowed it's frost on the fuselage-light frost,you have to be able to see the markings,and ice on the underside of the wing ,due to cold fuel-this is also limited in length..but again the upperside of the wing must be clear.It is suggested that ,if any doubt,you should check by hand the evental presence of clear ice.