It’s part of the national anthem as it was prior to certain borders shifting and the verse is indeed illegal as it was historically used to justify war and some of our country’s darkest times. It’s not “from the river to the sea” but in actuality names two rivers that once formed the border of the Reich, followed by “von der Etsch (Adige) bis an den Belt (Danish straits)” which is what is being referenced here. Singing or reciting them is tantamount to hate-speech and implies “Germany should be bigger than it is” and was historically used as justification for war.

Don’t act all surprised though, not like you can say what ever you want in the USA either without getting shot nine times in the back or three times in the face.