https://www.gamespot.com/articles/assassins-creed-odyssey-releasing-on-switch-in-jap/1100-6461769/

A "cloud version" of Assassin's Creed Odyssey will be released in Japan. The Japanese Nintendo Direct video presentation last week confirmed that a special, streaming-only version of the open-world stealthy-stabby game will be released on Nintendo's hybrid console in October.

As with the previously released cloud version of Resident Evil 7 for Switch, Odyssey will presumably run on cloud servers with only a small download file locally.