Don't want to write this by hand? GodCraft sets minecraft:uses_uniform_air_drag for you with a switch.Get GodCraftOverview
minecraft:uses_uniform_air_drag is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Changes how air slows the entity down so the same drag applies to its movement in every direction.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:uses_uniform_air_drag": {}
}