Taxonomy field allows the user to create new taxonomy terms. That’s great. But IMO there’s no need to open a modal to add new terms.
First of all, it’s requires a few additional steps (click plus button, enter the term, click save)
Secondly: the plus button that appears on hover looks a bit odd.
Finally, there’s no need to open the modal. If the term is not found in the autocomplete dropdown then the phrase that user has inputted could be automatically added as a new term. Select2 has such feature out of the box (See Tagging Support section on the Select2 Examples page).
Additionally: Select2 will be probably added to the WordPress core as a default UI for taxonomy terms. It was put on hold only because of some accessibility issues. And I believe it will work the same way as I suggest because in the current UI taxonomy terms are also added without opening a modal. If the term doesn’t exist in the autocomplete dropdown, it’s automatically created.
Thanks for saving/loading terms feature! I was just about to implement such a feature then I found that ACF has it out of the box 🙂