I commented on this a few weeks ago. There are two future components under development that look to support selecting multiple options in a dropdown format.

Search textbox: would be supported under the more customizable Combobox element

Select Multiple: both the Enhanced Select and Combobox plan to support this

Combobox: https://open-ui.org/components/combobox.explainer/

Enhanced Select: https://open-ui.org/components/customizableselect/