The answer has to be based on common sense and you don’t need to refer to websites or books. People including some cat owners believe that it is okay to lift a cat off the ground by his/her scruff (the loose skin at the back of the neck). They believe this because mother cats move their young offspring around to safe dens by placing the scruff of their necks in their mouths and carrying them while they dangle passively from their mouth. The method invokes the passive kitten response.
There is a world of difference between the two. Young kittens are small and light. The reason for moving the kitten is for safety reasons. It is natural and necessary.
A person grabbing an adult cat by the scruff and lifting him off the ground for some peculiar reason is quite possibly animal abuse and a crime under the relevant animal welfare laws of the country or state where the cat owner lives.
It is likely to be painful for the cat. There is no need for it because there are alternative, more humane and comfortable, ways to carry and move a domestic cat.
It could be more than painful. It may harm the cat. If done regularly it may also harm the relationship between owner and cat.
I have seen cat abusers on YouTube handling cats this way which tells you what I think about it. I remember seeing an idiot brutally holding his cat by the scruff. The cat was complaining and screaming. The man was having fun and videoing himself. Of course YouTube admin. failed to take the video down. This is ironic because Google who owns YouTube complain when a website criticise animal abuse through pictures (one of my articles).
The obvious point that any normal person would make on this subject is that you don’t need to carry a cat solely by the scruff of the neck.
There is an exception I believe. I have carried my cat by placing my left arm and hand under his body and gently grasping the scruff of his neck with my right hand. This pacifies him and makes carrying him easier. When might you do this? When your cat is playing up for some reason. For me, before I constructed a garden cat enclosure I used to go outside with him on a leash. He would be very reluctant to go back inside so I had to carry him and pacify him simultaneously as he was born feral and at that time could be a handful.
Also there was an occasion when my cat escaped the home and was under a bush by the front door. I had one option: to grab him by the scruff and drag him up and out. As soon as possible I supported his body with my free hand.
Other than under exceptional circumstances lifting an adult domestic cat by the scruff is unacceptable. Lifting kittens by the scruff is less problematic because they are light. However, I believe, it should only tried when necessary – for a specific reason.
P.S. The photos that make up the montage are in the public domain.