Kyouko still uses a flip phone, but others had been using smart phone for a long time, even way back in 2015 where Kyouko & Ren are playing the Heel siblings, Ren is already shown using a smart phone since way back, but if my memory is correct, it was Yashiro who was 1st person introduced using a smartphone.
true. the author should have mentioned the change somehow. maybe through an ad they were filming with a character we know promoting smartphones... that doesn't take many panels and would have been a smooth transition. even if the time within the story didn't progress much since the beginning... ( ̄∇ ̄")
Okay, I went and did a quick back read and found it was Shou who 1st started using a smartphone in chapter 185 watching Kyoko's interview with Ren on that Tsukigomori's party where Kyoko was dressed up by Kijima, then followed by Ms Woods (the "lover" of president lory), then Yashiro on chapter 216 then Kyoko's mom on chapter 218 and Ren on chapter 221.
I swear they were using flip-phones like 2 ch ago now it’s touch screens and Instagram