isInGameMilestone property
Some milestones are explicit objectives that you can see and interact with in the game. Some milestones are more conceptual, built by BNet to help advise you on activities and events that happen in-game but that aren't explicitly shown in game as Milestones. If this is TRUE, you can see this as a milestone in the game. If this is FALSE, it's an event or activity you can participate in, but you won't see it as a Milestone in the game's UI.
Implementation
@JsonKey(name:'isInGameMilestone')
bool? isInGameMilestone;