CurrencyDenominationsForm

That would probably make the most sense, although it could also just be a separate add form. The current d7 version just uses a very standard add/edit/delete setup, but it also isn't a config entity.

In short, whichever you feel is easier/better is fine with me.

Here's what I could find that I think need work:

• Got an error when trying to save a denomination
  • Drupal\Core\Entity\EntityMalformedException: The entity does not have an ID. in Drupal\Core\Config\Entity\ConfigEntityStorage->save()
• The $name_field variable is defined in both the main form build and in addmoreCallback() but never used.
• The "Remove one" button makes it difficult to manage denominations in the form. Instead, there should be a remove button for each denomination, that way if you need to remove one that's higher up in the list you don't need to remove and re-add all the others that come after it.
• The fields should be required.
• I noticed there's no currency field like in the screenshot, is that no longer in the plan?
• Since the addmoreCallback() method is used for both the add and remove actions it should have a more generic name, current one indicates it's only for when something's added to the form.
• Nothing's calling the validateCurrencyCode() method.
• Add some descriptions on the fields, it's not clear to an administrator what the fields are for.

Looks good.