Don't want to write this by hand? GodCraft sets minecraft:persistent for you with a switch.Get GodCraftOverview
minecraft:persistent is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Keeps the entity from despawning when players are far away. Add it to important mobs such as quest characters or bosses.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:persistent": {}
}