I found this lua snippet when searching for how to unbind default Roblox keybinds, and then did a bunch of searching for moveBackwardAction to find documentation for all the default actions but found no results.
local ContextActionService = game:GetService("ContextActionService")
ContextActionService:UnbindAction("moveBackwardAction")
It’s in the ControlModule in the default control scripts. There’s also a method for unbinding all of these at once, if that’s what you’re looking to do.