Glancing over the fan wiki it seems like it might be implied, but not openly stated.
As for what is open:
Trivia
On the Global server of Project SEKAI, Mizuki is referred to with both "she/her"and "they/them" pronouns, though it is unknown which Mizuki personally uses. When possible, Mizuki's name is used in place of pronouns entirely.
On the Japanese server of Project SEKAI, Mizuki typically uses the masculine first-person pronoun Boku (ボク), which is most often used by immature or young boys, though it is also used by tomboyish girls.
