I don't have access to the old manuals now, but I vaguely recall that it's written somewhere in the FCTM. Something along the lines of engine anti-ice should be on when operating in icing conditions or near CBs. Does anyone still have that reference?
Separately, the new FCTM does have a lot more write-ups about icing and how it can occur - Ice crystal section. In the region where I operate, very often even when the OAT is -50 degrees Celsius we see ice forming on the wipers after a while. Maybe the new procedure is to cater for possibilities like this.