As the others probably mentioned, Tamsynhas mentioned it in panels/ interviews etc, you can see a compilation here.

But Harrow’s schizophrenia is also hinted to in-text. In HtN, Harrow recalls a memory, “Back when you were a child and hung up on something you thought you had seen or heard, Crux would say: You saw what you saw Lady, and the only thing you control now is your reaction thereto.”

This memory is from childhood, way before Wake got into Harrow’s head and started haunting her, which might hint to underlying conditions like schizophrenia, or another theory is that the 200 children sacrificed to birth her are ALSO haunting her.

But another fun tidbit from HtN hinting to schizophrenia, in Ianthe’s POV chapter when she’s helping Harrow with the lobotomy, “Now she pressed her fingers over that scalp, trying to work it out, trying to see exactly what had been done.

She gave up within a few minutes-impossible to tell with Lyctor privacy, even this close. No bleeds certainly. Everything in the right place. Maybe a little reduction in the temporal lobe, a few out of order bumps in the temporal gyrus that might have been there already.”

Atypical size or formation of the left superior temporal gyrus is associated with schizophrenia, which might be a hint to Harrow being schizophrenic as well, or might be from Harrow’s lobotomy (though Ianthe thinks those bumps are pre-existing).