A long term calorie restriction that is so great has a few beneficial effects, i.e. a lower risk of cancer and a good probability of a longer life, but only if you live in very sheltered conditions, e.g. when being a lab rat or mouse or when having servants who would do everything for you.

However there are significant undesirable effects, caused by a reduced muscular mass and adipose tissue. You would become one of the weak and frail old people, who easily fall and break some limb. The resistance to any infectious diseases is also likely to be very weak.

Unfortunately, many choices that you could do about your lifestyle have both advantages and disadvantages, so it may be difficult or impossible to find an optimum. Few decisions are simple, like the decision that smoking is bad.

Someone with a sedentary lifestyle would need to eat around 1200 to 1300 kcal daily to achieve such a calorie restriction, e.g. 2 meals per day of around 600 kcal each. Of this amount of calories per day, around 400 kcal should come from protein, around 350 kcal from fat and around 450 to 550 kcal from starch. It is feasible, but it may be difficult to not be hungry between meals. Sweets would have to be mostly excluded, as they would prevent satiety and they would cause hunger soon after a meal. Protein would have to be provided mostly by pure sources, e.g. chicken/turkey breast, gelatin or protein powders, to avoid extra calories.

Most people severely underestimate their ability to become accustomed to eating much less than they currently eat, but even so I am doubtful that it is possible to enjoy eating so little as required by a lifelong 30% calorie restriction.