That is at least partially true. Over time, my reading is that only the very devoted can extend a real keto diet. Most go through modification or abandonment. HOwever, my memory is that there are some relatively long term keto diet studies.
It seems that keto diets are perhaps good for some people as a weight loss tool and possibly for other conditions .....and generally imiplemented on a temporary basis or included as dietary habit/practice in cycles.
Journal Alzheimers Disease and Dementia just reported that diets low in BCAA's increase risk of dementia.
Dementia, Alzheimer Disease Risk Increased With Lower Branched-Chain Amino Acid Levels - The Clinical Advisor