Don't want to write this by hand? GodCraft sets minecraft:uses_legacy_friction for you with a switch.Get GodCraftOverview
minecraft:uses_legacy_friction is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Uses the old, incorrect math for friction_modifier. It is added to older content to keep it working the same; don't use it for new entities.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:uses_legacy_friction": {}
}