Don't want to write this by hand? GodCraft sets minecraft:air_drag_modifier for you with a switch.Get GodCraftOverview
minecraft:air_drag_modifier is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Changes how much the air slows the entity down while it moves. Lower values let it glide farther, higher values make it stop sooner.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:air_drag_modifier": {}
}