You say that, but I’m based in Germany and had stickers of this art when I was kettled and searched by police on a 1 May demonstration, and I was allowed to keep them (actually being caught in the act of stickering would have been an offence). To answer the OP, the main slogan that has faced criminalisation is “from the river to the sea”. Aside from that and explicitly expressing support to proscribed organisations, there’s not too much that will land you in serious trouble, though there’s no consistent standard and the courts decide on things quite unpredictably for now. Last year there were deportation orders for foreigners on the basis of occupying a university building, rather than a certain slogan.