multiselect.html.erb

<div 
  data-controller="multiselect"
  data-multiselect-selected-items-value=[]
>
  ...
</div>

multiselect_controller.js

import { Controller } from "stimulus"

export default class extends Controller {
  static values = {
    selectedItems: Array // https://stimulus.hotwire.dev/reference/values#types
  }

  updateSelected(customSelectedEvent) {
    // add the selected item to the selected items array
    this.selectedItemsValue.push(customSelectedEvent.detail.value) 
  }

  updateDeselected(customDeselectedEvent) {
    // Find the index of the deselected item in the selected items array
    const index = array.indexOf(customDeselectedEvent.detail.value);
    
    // If the deselected item is in the array
    if (index > -1) {
      // Remove the deselected item from the array
      this.selectedItemsValue.splice(index, 1);
    }
  }
}