You can set a ComboBox's button's icon's aria-label via buttonIconProps, but screen readers don't read that value, presumably because the parent button is set to aria-hidden="true". ComboBox hard-codes that value to true, and there doesn't seem to be a mechanism to change that.

Can we expose a way to set aria-hidden to false?

Also, is there a reason why the aria-label is being set on the button's icon, and not the button itself?