Unlike stuff like Wormadam where it's just locations and the Eeveelutions from 4th gen, I'd argue that the seasons is a huge mechanic and since there's a Pokemon that revolves around them, it'd be odd if they took it out.
I certainly hope they leave it in and have more Pokemon that change depending on the season/the different rate of appearance. After all they said that they're gonna have a wide variety of Pokemon in the grass to try to keep you from seeing a bunch of the same ones.
