ChunkPopulateEvent states that it is fired when a new chunk has been populated, due to 1.18 this isn't the case anymore and when a pre-1.18 world is upgraded the event is fired for chunks that get their y0 to y-64 blocks replaced/updated. This wouldn't be that big of an issue if it was documented and if we could tell if the chunk is new or not similar to ChunkLoadEvent#isNewChunk

A case example is you have a 1.17 world, you use the event to remove all spawners in the world when they are added, then you upgrade to 1.18 with the same event code, it'll remove all spawners in the new y0 to y-64 zone which is fine but any player placed spawners above y0 will be removed as well since you never expected the event to fire.