What is the standard way to secure flex duct in an attic?
That duct is not done properly notice how the duct is collapsed. Webbing can be used I have used plumbers tape (a metal strip with holes) at the first support after a rise I will screw a saddle of scrap ductwork to keep the duct from collapsing. I have used webbing but always think it will separate over time I usually melt the holes to reduce the chance of this but still make a saddle on the rise to spread the force out then support it regularly so it won’t collapse but nylon strapping is ok. I would want to fix that collapsed section a section of single wall duct like dryer duct opened up could make several sections of saddles.