It's not illegal, but if you're hiding content from Googlebot because you don't want it to derank that page, once they crawl it with an agent that hides the fac

It's not illegal, but if you're hiding content from Googlebot because you don't want it to derank that page, once they crawl it with an agent that hides the fact that it's Google, they will then derank your whole website. So it's not illegal, but if you think you're beating Google by doing it, they're well aware of this workaround and will punish you harder. If you don't care about ranking, then no biggie. But why would you manipulate your web server like that if you weren't trying to rank.