irredentism and nationalism is an important part of human political behavior from current day South America to Nazi Germany. For one, I would argue it is so natural to player psychology to recognize former possessions as uniquely special targets that to face an AI deaf to its importance instills confusion and lessens immersion. AI should respond to human psychology, anything else looks artificial and designed, even if it is a competitive threat, it isn't organic.
Some civilizations maybe excluding something like the Yor should treat it former territories as special targets for reclamation, even in cases where this policy brash and not entirely realist, as not all actors are emotionally numb in foreign policy and this does lead to mistakes and diplomatic intrigues.
Game mechanic possibilities
If you attack a foreign civilization and steal their planet they have a special interest in reclamation for x turns.
Part of peace agreements in territory exchanges,
Finally, there are a myriad of historical treaties and examples I hope can be used for inspiration in the game.