Before the introduction of unreachable task handling (TASK_LOST or TASK_UNREACHABLE), Marathon generally expunged tasks that were being reported lost or unreachable. The introduction of that configuration option changed the default behavior so that these tasks were kept in state and not immediately expunged.
We added a migration option for 1.4 that allows correcting specific configs to use UnreachableStrategy(0,0) instead, but we didn't change the default that is applied when a new app or pod is created.
State of thinking is that we should correct this and re-introduce a default of UnreachableStrategy(0,0).
We decided not to make UnreachableStrategy configurable.