Its closure is also mentioned in the text: "After the 2019 closure of the Puttgarden-Rødby service between Germany and Denmark and the seasonal Sassnitz-Trelleborg route linking Germany and Sweden in 2020, the Intercity is now the last one running."

Ah, sweet memories. Grumpy German border guards boarding to loudly demand papers, but if you were slow to pull the ID card out they just kept walking. Semi-open border policy ;)

During the Cold War, the train on the Trelleborg-Sassnitz ferry went to West Berlin ... through East Germany with no getting on or off over that stretch.

BTW. I was on the Hamburg-Copenhagen train on the Puttgarten-Rødby ferry in 2015 during the height of the Syrian refugee crisis. I had changed my booking to earlier trains because of expected delays. It and connecting trains were packed, as were the train stations, also with immigration officials and volunteers. Brings tears to my eyes to this day.