Don't want to write this by hand? GodCraft sets minecraft:can_fly for you with a switch.Get GodCraftOverview
minecraft:can_fly is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Marks the entity as a flyer, so its pathfinding does not need solid ground below. Use it together with a flying movement and navigation component.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:can_fly": {}
}