Don't want to write this by hand? GodCraft sets minecraft:transient for you with a switch.Get GodCraftOverview
minecraft:transient is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Stops the entity from being saved, so it disappears when the world is closed or the chunk unloads.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:transient": {}
}