The restriction on selecting flap in a turn (usually only a recommendation, but some instructors train that way) may be valid for aeroplanes that don't have asymmetry protection for the obvious reason that a surprise asymmetry could make life temporarily miserable if it happened in a turn. As has been mentioned, a descending turn won't impose 'g' loadings likely to cause any problems with either the gear or flaps. If the turn is balanced there won't be side loads, so the gear thingy must be unique to a few types, none of which I have flown.
If the POH or AFM says don't activate the gear or flaps in a turn then that is a clear limitation - period. If some other WRITTEN instruction from the owner or operator wishes to add additional limitations, it is incumbent upon the pilot hiring or being paid to fly the aeroplane to obey those instructions - whether they have any real basis or not.
But if it's just some silly old tosser's pet theory, ignore it if you know better and are certain that no instructions to the contrary exist.