I guess PC Gamer covered this already, but Codemasters community manager justbiglee wrote in a post on reddit that mod support for the brand new rally sim meant to succeed Richard Burns Rally won’t be implemented by Codemasters due to licensing issues:
Meanwhile, due to the game using the EGO Engine (as all previous Codemasters games have this generation), mods have already started popping up. First, our very own camera mod was done using a simple XML editor specific to Codemasters games:
Next, custom skinning appears to be relatively simple as the models haven’t changed since DiRT 3, and a lot of kids are just importing skins they made for the 2011 title into DiRT Rally, as the file structure is the same:
And finally, people have started messing around with importing models on pre-existing sets of physics:
Similar to Richard Burns Rally a decade ago, even if the game isn’t open to modding, people within the community will figure it out.