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Well, it can when not done right.

But, as vegans will be glad to tell you, it's not a problem once a person gets up to speed in balancing their diet.

That being said, since not everyone that goes vegan does their research and learns how to do that, it is definitely a thing.

It isn't just vegans though. Vegetarians can and do run into it, but they tend to be less exclusionary of dairy and eggs, so you don't run into it as often.

I'll also say that the effects aren't severe unless the person is just eating poorly, which is also true of people that aren't purely plant based. Eating shitty causes decreases in function, period. This includes mental slowdowns. Yeah, you get the iron and b12 eating meat, but if that's all you eat, that has its own drawbacks. And when you're eating plants and meat, but aren't eating a diverse, varied diet, you run into problems.

Hell, you can be eating a varied diet and run into deficiencies anyway, if you aren't paying attention. It's harder, but it's not impossible.

In other words, the headline is clickbait. It just isn't unfounded clickbait.

Out here in meat space, the vegans I know have no issues because they've been vegan for a long time and have learned how to keep things balanced.