According to Superman #400, Superman disappeared sometime during the 21st century. People fully expected him to return so they kept setting a place for him for Miracle Monday dinner up until at least the 60th century.
https://www.supermanthroughtheages.com/a/400/dinner/?page=9"The Ghost of Superman Future" does state that between journalists finding a 130 year old Superman and the time the nations of Earth saw him again, he had passed from legend to myth. That would suggest that Superman did live at least several more centuries, perhaps a millenium given the 'myth' statement, beyond the 21st century.
https://www.supermanthroughtheages.com/tales2/ghost/?page=8In DC One Million (Post Crisis), this was further elaborated upon so that Superman still left Earth towards the close of the 21st century. But in this version, it was said he had lost his reason for staying on Earth since his wife, friends, and family had all passed away. So, he set out on adventures to the Edge of the universe.
All of the above explain how Superman could be alive in the 30th/31st century and not be interacting with the Legion or the United Planets. He simply had left Earth and its area of influence on some personal quest.