The docs for the declarative settings API contain the following sentence:
When new entries need a multi-field input or validation beyond a single inline text field, open a Modal from addItem.action.
(/Plugins/User+interface/Settings#Lists+with+a+form+modal (I can’t include links))
That sounds to me like there is a simple way to add a new item without a modal, if the only thing I need is “a single inline text field”. I sadly couldn’t find one. Did I miss something? Is it still wip?