In Legion, the player wasgiven a short questline early in the story in which they are rewarded with an Artifact Weapon. This weapon came equipped with slots to upgrade it and a skill tree, meaning the weapon remained relevant to your progression for the entire expansion. You spent a little less than two years developing this weapon. Growing its power through the acquisition of new items to slot into it and more artifact power to unlock more of the skill tree. It is not an exaggeration to say that this weapon had a bigger impact on your damage output and survivability than your class/specialization's own natural abilities.

And at the end of the expansion, the skill tree was completely removed. All of the power you had come to rely on, that completely redefined how you played your class, was stripped from the game. That is what "borrowed power" means. The Artifact Weapons were not the first time WoW has done this, and they wouldn't be the last time, but they are the most egregious (or so I'm aware—I haven't bothered with BFA or Shadowlands).

I can't think of anything comparable to that in FFXIV. The closest might be those infinitesimally rare equipment drops from the old Diadem, which were made unavailable when it was reworked into a gathering zone for Ishgard Restoration.