It is important for animal advocates to be intellectually honest about the balance of the pros and cons to health from abstaining from animal products (being vegan). If they are not intellectually honest, their arguments will be discredited and discounted in the future by the general public.
Unfortunately, I think some of the popular medical doctors advocating for plant based diets, including Neal Barnard and Joel Fuhrman, have gotten a reputation for only presenting one side of the evidence. Similarly, I am not sure if the 2018 documentary Game Changers helped or damaged the arguments for plant-based diets since it was widely reviewed as insufficiently sober in its handling of a complex subject (https://www.menshealth.com/nutrition/a29067926/the-game-changers-movie-fact-check/). If I am wrong about this I would be happy to be corrected.
I have heard arguments recently that one of the most important components of healthy aging is minimizing sarcopenia, and this cannot be feasibly done without large doses of protein, including animal protein, spread out throughout the day (https://peterattiamd.com/donlayman/). These arguments are being advanced by respected academics, who are also arguing against school lunches going plant-based.
If animal advocates are going to suggest that the public abstain from consuming animal products, I believe they should address the sarcopenia point, and other similar points, directly and credibly. If this has already been done, I apologize, and would request that someone direct me to those materials. If it has not, I would like to suggest that this is a void that would be helpful to fill.
The quality of research is already there: See my comment here.
I'm not aware of any newer studies on creatine after the original one that spawned this belief; I will point out the original studies were likely aggressively p-hacked, showed impossible effect sizes, and make very little sense because brain creatine levels are not substantially different between vegetarians and vegans.
Still, it's not completely out of the question there's a real effect. We still want to avoid the fallacy of the one-sided bet. It's definitely possible that vegetarianism causes a drop in . But given vegetarianism seems to lower blood pressure, LDL cholesterol, and obesity rates, I'd be willing to put money on the exact opposite. A healthier kid is a smarter kid, and vegetarianism seems to have positive effects on health in general.
If you're still skeptical, over-the-counter creatine is extremely cheap (and has benefits for physical performance anyways). Trying to get rid of LDL cholesterol, on the other hand, is pretty hard.