A Twitter account that tracks the Japanese trademarks spotted the trademark late last week.
Still, it would appear Nintendo has made good on its promise to get more units into consumers' hands. However, a recent tweet by a Japanese licensing and trademark bot has set off rumours that Nintendo is exploring possibilities of either bringing back the GameBoy or revitalising it for a new generation of players. The SNES Classic Mini just recently hit the stands, and although it has 21 games on its own, a new hack is able to load up to 150 games in the library.
It doesn't specify the name on the trademark, but it covers a variety of "home video game console" programs as well as "programs for smartphones", "smartphone cases" and "smartphone covers", as well as "key holders", "necklaces" and "watches".
Nintendo saw fit to publish a series of interviews leading up to the SNES Classic release, and while they were timed well on the Japanese site, the English versions needed to catch up.
So it could be that Nintendo is simply attempting to market the design of the Game Boy rather than re-release the console. Since then, Nintendo has released increasingly sophisticated mobile consoles, culminating in the Switch, its first true hybrid game system.
Back in 2005, Nintendo released a smaller version of the Game Boy Advance called the Game Boy Micro.