[2022] GGG, can we get some clarification on Gravebind and Herald of Ash (and other on-kill effects)?

Herald of Ash requires you deal a killing blow, not just kill. This isn't stated explicitly because it's implicit in the fact that it requires overkill damage, which is only possible from a killing blow, and stating it explicitly would make Herald of Ash awkward to read in a way that could add to confusion. This might get re-examined at some point, but care would have to be taken not to sacrifice clarity for new players reading Herald of Ash and trying to understand just that skill, to aid clarity around a specific very niche interaction with a unique item most players will never see.

As suggested in this very post, "your overkill damage" instead of "the overkill damage" would make it clear on a careful reading and add zero confusion.

That would absolutely add confusion for some players.

"While you have this buff, if you kill an enemy, other enemies near them will be burned based on your overkill damage."

"your" is really awkward there, and makes the "your overkill damage" part sound like some keyword phrase which is a property of the player character rather than part of the killing event. It feels weird to specify the overkill damage is "yours" there, and not entirely intuitive that overkill damage conceptualy is "yours" when it's fundamentally defined more by the enemy it's happening to than you being what dealt it.

That doesn't mean the current wording is perfect or no improvements are possible, but they have to be carefully considered, and the text is serving it's primary purpose - describing just Herald of Ash - very well, as evidenced by the fact that it's been that way for ages and people have generally been able to easily understand the skill without much trouble. So putting effort into working out a new wording to improve understanding of a very niche interaction with something outside the skill itself, has a low priority.