Currently, v1.1.0.42, all you need to turn the spawn-despawn ON is here:

Source: https://community.bistudio.com/wiki/Arma_Reforger:Scenario_Framework#Dynamic_Spawn/Despawn_2   Critical: If you're running a Coop/PvE/CombatOps mission, in your World Editor, through the Hierarchy column area, click on the GameModeSF and then right-click over its property called SCR_GameModeSF. Click on Change Class and select SCR_GameModeCombatOpsManager and turn on the Dynamic Despawn; If you're running another kind of mission, probably you should leave the original component and only turn on the Dynamic Despawn checkbox;