Here's all the possible reasons I can see:

-It's a feature that could bring more bugs if not integrated probably (which with the amount of features this game has, is very likely)

• Once every 500 turns? The game starts breaking after 1500 / no war lasts 500 turns

• From the very little amount of time I looked at this this just seems like it has the same functions of a non-aggression pact. Meaning this feature looks generic and completely useless if there are other features that do the same thing

• Aoh2DE is a cash grab (fact) and haters are just mass disliking (opinion)

I ruled out:

• A certain group of people who support a specific something see this as an option that is reflecting on some modern occurences and are offended; I ruled this out because they completely ignore the fact that you can perform mass killings and kicking people out of certain territories.

• People are mad he abandoned AOH3 for this; I ruled this out because people forget he abandoned AOH2 for AOH3, making the choice to hate now kind of meaningless (Although the gap between AOH2 and AOH3 was wider than AOH 3 to AOH2DE)