He or she, or nowadays they for some transgender or non-binary people.

Obviously Aran'gar is a different case from transgender people in the real world, and hardly someone they would identify with, or would want to. But that doesn't mean it makes sense to use 'it'. And yes, I'd agree with fionwe that if you use 'it' for Aran'gar, that suggests you'd do the same for transgender or non-binary people in real life. That suggests that people who are for whatever reason neither fully male nor fully female (or don't think of themselves as such) are somehow not human.

'Misgendering' Aran'gar by continuing to use 'he' until the end - or, as mook seemed to be suggesting, by using 'they' - would have been one thing. What with the saidin use, there's an argument to be made for using 'he' within the WoT universe, even if it's jarring since the character so clearly views herself as a woman later on. But dehumanizing her by using 'it' is something else.