Couple of things I noticed from
Friends and family who went full vegan. At first, they feel incredible. Energy boost, fat loss, even no need to wear deodorant. Eventually, things such as fat loss, etc reversed. Inflammation became another big issue probably due to high grain consumption. After several years, they began to feel run down, lost strength, and began to crave meat 24/7 as if their body needed it.
Some of those people just added fish or eggs and solved a lot of problems. Others kept the clean eating principles of a plant based diets and added local organic meats etc. 2 others switched to full keto.
The conclusion they came to was it was great at first to detoxify themselves and have more strict and disciplined eating habits. However, only one of the 11 people I knew felt it was sustainable long term.
Now there are people who have been vegan for years and been fine. I also personally know 2 people who have not had anything more than a trace carb and close to 20 years. I would imagine the answer lies somewhere in the middle