While a plan to exile U.S. citizens, even if they are all convicted criminals or if that only applies to naturalized individuals, is obviously unconstitutional, the Trump administration has already carried out illegal deportations to El Salvador — so it's very prudent to take these remarks seriously. It's time for courts to intervene before American citizens are removed from the country.
The Trump administration is looking into this idea to address crime from all over the country and will go through with it only if the law allows — especially because no one wants to create a precedent that could be disastrous in the hands of Democrats. It's likely that the proposal will focus on denaturalizing and stripping citizenship from violent criminals despite hysteria from the left.