Has anyone tried using this in Plone 5? It looks like Plone5 comes with an ancient version of select2 (2012!) that is pre 4.0 so it does not have some useful options. In particular dynamic tagging on a single value tag Dynamic option creation | Select2 - The jQuery replacement for select boxes. I'm under the assumption that trying to include a newer version of select2 will break the mockup patterns using the older version no?
IIRC from one of the virtual sprint chats in 2020, @MrTango put quite some effort in it himself to try to update select2 to the latest version.
But if I paraphrase him correctly, there were so many changes and work arounds needed on Select2 for the integration and to let it work properly with the CMS that he estimates it's quicker to create a custom related items frontend component from scratch in one of the available generic frontend libraries like Vue, Svelte or React.