You need Mafia 3 ScriptHook: 
https://mafiamods.com/mods/mafia-3-scripthook

Installation:

- Copy "noclip.lua" to "Mafia III\scripts" folder.
- Run Mafia 3 and Mafia 3 ScriptHookLauncher
- Press F1 to load script

Controls:

U = Up
I = Forward
O = Down
K = Backward
J = Left
L = Right

Bugs:

Lincoln keeps falling, so you must hold noclip keys.
If you hold noclip keys long, your game may crash.