Are frayed cables of Apple lightning cord or Macbook Power Adapter covered by warranty?
According to Apple One (1) Year Limited Warranty – Accessory:


  This warranty excludes normal depletion of consumable parts such as
  batteries unless failure has occurred due to a defect in materials or
  workmanship and, damage resulting from abuse, accident, modifications,
  unauthorized repairs or other causes that are not defects in materials
  and workmanship.


This is the term that Apple store or service providers often rely on to reject warranty of cases like you mentioned. They could say it was caused by abusive usage behavior, or by accident thus violate the warranty term.

If you are specifically looking to confirm has Apple ever issued a recall/repair program for frayed lightning cable then the answer is No.