I believe this has been suggested multiple times over the years; the usual response is that it would take too much effort to set up the cars for it (and possibly also too much computing power, though I no longer remember).
I wouldn't expect real time terrain deformation like Spintires, but a dirt/mud decal texture system wouldn't be a far stretch. I'm certain the devs will add the texture aspect eventually as a last visual update pass, more important updates will take priority of course.
Man... we have been requestion decals for the last 3 years. They could do everything, from scratches on metal to mud on the cars. Yet the devs have never acknowledged the idea as far as i know
There was a time when we didn't have all the separate vehicle engine sounds, scrapes, surface particles/tyre sound variety we do now. It only took 5 YEARS but they delivered on some of the smaller details that the community requested. The real work has been done on overhauling the physics, adding a career mode, maps, and so on. I'm sure once they start to wrap development they can focus on some of the fun stuff. The drive is not likely to end anytime soon, so buckle up and have a little faith. These guys are passionate people working on their handcrafted baby.
Combine Spintires with BeamNg and you'd have my favorite game of all time, but it would probably be really difficult.
The problem is non pre-determined textures, which is the only way it'll be implemented. That's the big point the developers have historically brought up when debating this.
It can be done many ways. While not exactly technical I can imagine either with a smaller detailed texture to be alphaed in with random orientations or a full vehicle texture like a "skin". The effect texture can be triggered in by how much each node has been explosed to a given surface type and transfer that in a average across an area of vertexes on the model. The real take away is if the Mudrunner dev blogs taught me anything, everything you see is all an illusion. You can create very convincing effects with little imput and resource requirements.