> 4-5 chicken breasts is around 120g a day

Bad math? Per USDA standards, a single boneless skinless chicken breast has ~54 grams of protein; so 4-5 would be ~200-250g of protein.

Because that's grossly outside the norm for the general public, you're not going to find any evidence to support the idea it's a healthy amount for a typical person to consume for a long period of time. And likewise, you'll find little evidence saying what negative consequences it might have, if any.

You're welcome to make whatever assumptions you want to in that case, but there's not a lot of ground for anyone to convince skeptics who disagree with them. It's tenuous assumptions all the way down.

Regardless, in the real world, that also represents 1200-1500 calories of absurdly (mind-numbingly) high-satiety food and quite a lot of slow digestive bulk. Most people simply wouldn't be able to consume that while also eating a varied diet that provides them with adequate long-term nutrition. So it's probably a pretty bad idea for them to dedicate themselves to it, unless -- like some athletes and gym bros -- they have the further discipline to also stuff themselves of all the other stuff they need to eat while also not eating so much that they become overweight. Do you know many people like that? I'm not sure I've met more than a handful in my lifetime.

Whatever the impact of the very high protein consumption itself in some abstract theoretical kind of way, which we're far from having evidence into understanding, it's just terrible advice for the general public because of the secondary effects we might reasonably expect in practice.