I needed the same thing so i used Selectize inputfield single for the master select and then a select multiple on the dependent select, then i initialized the 2nd one using Selectize JS; the JS is not so hard, but you do need to have a custom select multiple that has a data-parent attribute on each option; those are filtered to match the selected option in the 'parent' select. Eventually this will make it into InputfieldSelectize somehow, but for now it's just done by adding JS, courtesy of
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