Sony’s controversial PlayStation Network requirement for its PC games is once again in the spotlight after the company delisted the original Horizon Zero Dawn on Steam and the Epic Games Store and added the PSN requirement to the upcoming single-player remaster.
For now
Well they have structured the Remaster as a DLC upgrade that requires the base game license.
So they would have to delist their new listing and create an entirely new release.
Honestly 99% chance they did it this way to preserve the review history of the original game on the Remaster.