Despite the continuous declaration that posts related to this one are duplicates that have been resolved, there is a consistent trend that it is currently implausible to lose Zombie Pigman aggression after they are attacked from one of many different possible sources. This issue has been observed in the 1.14 Pre-Releases, among other snapshots.

Waiting out the 20 to 39.9 second aggression timer and leaving the 40 block detection radius of the Pigmen will not remove their aggression. Alongside this, the continuous spawn of additional pursuing Pigmen that 'refresh' the aggression timer make it impractical for a player to isolate and slaughter every single aggressive Pigman that has been loaded and unloaded in the Nether.

After testing Pigman aggression in 1.13.2, for comparison, it is noted that players do not even need to leave the 40 block detection radius of aggressive Pigmen, but rather only wait out the aggression timer before the Pigmen become passive once again.

It is understandable that this may not be a commonly noticed bug, but it makes the Nether near inaccessible once a single Pigman has been attacked, and has appeared to persist indefinitely.