No. For automobiles especially, your car's warranties aren't voided unless your modifications contributed/create a problem in the car. For Tesla, perhaps they could argue that removing the modem means they can't diagnose effectively because it's offline and not reporting any errors, but if a motor died they'd still have to fix it despite the modem being ripped out.
That may be illegal just like it's illegal to tamper with consumer electronics/HDMI devices
For automobile manufacturers to use the DMCA in this way they would have to make a case for some kind of copyright infringement that would be possible if the cellular modem was removed. Not impossible, but it would be a stretch for now.