Root Letter: Last Answer is now up for pre-purchase on the Japanese Switch eShop.
The game is currently being offered for 4,980 yen but pre-purchase will guarantee a 10 percent discount until December 19. File size is at 5.2GB.
Root Letter: Last Answer is slated for release on December 20. Sadly, no English language option is available for the game.
Check out the screenies below (click to enlarge):