RSC removed the link to the mod, but you can do a web search via google for slicks for all cars and will find it as the first or second hit.
In my opinion, slicks for all cars, if server optional, would be a good thing for LFS. This allows players to compete on a non-slick tire server if they so choose.
The mod pretty much allows this (if the host has the slick tire mod, than other can join and use slicks online), but it would be nice to include it in the game.
Changing gearing and rear end differential locking rates is something rarely done on most street class racing, yet LFS allows this. On the other hand, most real racing with street cars is done with very sticky DOT tires which are only "technically" street legal, as they are not really intended for street use, or it is done with true slicks.
Rather than trying to draw a line as to which cars could have slicks, just let them all have slicks, but have a server option to disallow slicks, just as the servers can currently restrict which cars can run on a server.
What would be the harm of this scheme?