Considering the fact that the Assassin's Creed franchise more or less is about a ninja-like organisation that is all about assassinations, it's weird that no installation so far has taken place in Japan.
Fortunately, this is about to change as Assassin's Creed Codename Red was announced in September last year after a lot of previous rumours. While we do not know a whole lot about this title, it will take place in feudal Japan and is being developed by Ubisoft Quebec (who also made Assassin's Creed Odyssey).
Now it has been noticed that Ubisoft has been advertising on LinkedIn, looking for partners in Japan for "the biggest blockbuster of 2024". When this was noticed by Access The Animus on Twitter, Ubisoft removed the 2024 part, but in Japanese it still says (translated with Bing):
"We are looking for sponsor companies to tie up with this action RPG game scheduled for a release next year."
While nothing is confirmed until we've had an official confirmation from Ubisoft, this is still a pretty good clue on when to expect the Japanese assassins.