Ubuntu 23.10 (Mantic Minotaur) は 2024年7月11日 に EOL
"This is a follow-up to the End of Life warning sent earlier to confirm that as of July 11, 2024, Ubuntu 23.10 is no longer supported. No more package updates will be accepted to 23.10, and it will be archived to old-releases.ubuntu.com in the coming weeks."
Ubuntu 23.10 (Mantic Minotaur) reached End of Life on July 11, 2024 https://lists.ubuntu.com/archives/ubuntu-announce/2024-July/000303.html