Would it be free speech to write "go kill person x"? I think it isn't? So if the brackets are code for something illegal, why would it be legal to use them? (I'

Would it be free speech to write "go kill person x"? I think it isn't? So if the brackets are code for something illegal, why would it be legal to use them? (I'm not saying they should be illegal, as I am not 100% sure what they mean, but perhaps it should be investigated).