If you change a string field to use an Auto-Number format, the field is set to Read-Only, and Requiredness is ignored on any form in the Classic UI.

However, in the Unified Interface, the field remains editable, and if you try to save the record without supplying a value, and the field is required, then you must supply a value before saving.

The Unified Interface should behave like the Classic UI, by making the field Read-Only, and ignoring the Requiredness of any Auto-Numbered fields.