When did they say this?
Some stuff:
1. Kazuhisa Wada, the director and producer for the recent Persona spin-offs, said he couldn't announce a new title at the beginning of 2016, and that the year would be "one of preparation." He reiterated the preparation line at the end of 2016.
2. Naoto Hiraoka, Atlus bigwig, confirmed there was at least 1 new Persona game in development in December 2016. He basically confirmed the same in March 2017, when he spoke with GNN Gamer and said that plans for a new release in the Persona series were "in preparation."
3. DJ Taku Takahashi said, at the beginning of this year, that he worked on remixing Persona 5 music, which he said "might be included in the next game." His interview was stealthily edited to remove the Persona 5 mention out.
4. A P Studio designer, one who worked on DLC costume design for Persona 5, said she has been working on the design of costumes for characters in a new game. She said the designs might be "surprising" to fans.
5. The Atlus consumer survey released in February 2017 explicitly asked fans if they would want very specific titles, including stuff like "Persona 3 Dancing," "Persona 5 Arena," "Persona Q-2 and a remake of Persona Q," "Persona 1 HD console remake," "Persona shooting game," etc.
I mean, it's at least apparent that Atlus is heavily thinking about making spin-offs and remakes of Persona.