A very common scenario is that a user needs to filter a gallery/table/... based on a combobox but also needs to remove the filter or select all items in the dropdownlist. To (de)select all items is impossible with the current combobox and is a real limitation for a user friendly experience. Users expect the behaviour of Power BI where you can multiselect items:
When no items are selected, everything is shown in the report. In PowerApps however, nothing is shown in the gallery = inconsistent behaviour Power BI vs PowerApps
In Power BI there is an option to add a "Select all" in the dropdownlist which makes it possible to deselect or select all items with 1 or 2 clicks
In Power BI the state of the selected and unselected items are shown by a checkbox in front of each item in the combobox which makes it very intuitive
Please add this behaviour or add a custom combobox with this behaviour so we can create an integrated experience across the Power Platform. To select all or deselect all is in my opinion a standard functionality.