Don't want to write this by hand? GodCraft sets minecraft:dimension_bound for you with a switch.Get GodCraftOverview
minecraft:dimension_bound 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 going through portals to other dimensions.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:dimension_bound": {}
}