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.
@TrueAether It's Monna not Lynn. Monna has brown short hair and Lynn has long black hair. Monna's hair would be visible in the screenshot in the art, but it's not. It's brown short hair right at the top of the screenshot.
Here's a screenshot of the scene I grabbed from a video. Notice it's Monna standing to the left of Ellen.
Image is tagged Vocaloid on Pixiv but I'm not sure if this is a reference to a real-life death or a Vocaloid song. First impression is that this is wowaka but the upload date would be off by two months.
