Deleting ACF fields that are not longer used can be problematic. I do not have any experience with these plugins but I know where the problems can happen.
These plugins look at the field name or “meta_key”. The issue is that you might have multiple fields on different post type that have this same field name and this would mean that you’d delete content that you did not want to delete.
Unless you are sure that the field name is not used anywhere on the site I would not delete it. I, for one, know that I don’t always use unique field names.
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