There will be no proteins and fats necessary for the nutrition of the neurons of the brain, the rate of synaptogenesis - the formation of nerve connections - will decrease. Stop thinking and bring insanity closer, in other words.
Vegetarianism? (lacto-ovo?) Nothing special. It is more difficult to regulate a little calorie content, but physical activity can be increased. Pescaeganism is generally useful.
Strict veganism is harmful, it is harmful by the lack of nutrients in the diet that are not found in plants, but are found only in animal products.
But if you really want to, these Nutrients can be purchased at the pharmacy and entered in a convenient way.
With an unreasonable approach, there is a risk of deficiency of micro- and macronutrients. French fries and chips are also suitable for vegetarians.
But if you follow the basic principles of a healthy diet, everything will be ok.
A recently completed multi-year study comparing vegetarians, pestovegetarians, and meat-eaters found nothing special. Meat eaters have heart attacks more often, vegetarians have strokes, otherwise everything is about the same.
I made my choice , I prefer a heart attack, after it they rarely remain a vegetable.