GAME FREAK Game Designer Shigeru Ohmori was interviewed by IGN and he confirmed that Pokémon Ultra Sun and Ultra Moon will be the last main series Pokémon RPG games on the Nintendo 3DS. It was announced earlier in the year that a new Pokémon title was in development for the Nintendo Switch for release in 2018.

Below are Interview quotes:
"When we were making Pokemon X and Y, we really were trying to push the 3DS system to its absolute limits - which is what we thought we’d done. But when Sun and Moon came around, we completely redesigned the system, and actually ended up pushing the 3DS even further to what we thought was the most we could draw out of it."

"With Ultra Sun and Ultra Moon," explains Ohmori, "we’ve tried to eke that out more and really, really push the system to its absolute limits, and we’re now feeling that perhaps this is the maximum of what we can get out. So we’re really treating Ultra Sun and Ultra Moon as the culmination of our work with the 3DS system."

Source: http://ca.ign.com/articles/2017/10/19/ultra-sun-and-ultra-moon-will-be-the-last-pokemon-rpgs-for-3ds