Bug: unable to disable standard ligatures from Character and paragraph style options pane
There appears to be a bug in Illustrator 28.0 which makes it impossible to disable standard ligatures in the Paragraph Style Options or Character Style Options windows.
If you click the Standard ligatures toggle in this window, it will change to a dash and have no effect on the text. Discretionary ligatures has the same visual effect in the Options, but they can in fact be disabled here.

Both types of ligatures can be disabled in the OpenType panel and the style updated with Redefine Style. Standard ligatures shows as an empty checkbox, but if you then activate them in the Style Options you can no longer deactivate them.
Discretionary ligatures will be disabled, but may show as a dash if the style first was created using a font that includes discretionary ligatures. (If you create the style with a font that has discretionary ligatures, switch them off, then switch to a font without them and redefine style, this toggle will also show as unchecked):

