they're not officially included, mainly because there's no simulation of water or hydroplaning, so there's currently no need for them.
well actually they added water physics, and ive hydroplaned before. What does that have to do with semi-slicks. oh wait... you mean on road. Im an idot
There is a texture/model for them in the game files, but we haven't heard anything about their existence from the devs
Now I want someone to make them a mod, I would do it myself if I could figure out all the mess that is the wheels and tires system
I saw neilogicals video a few days ago of a dude making a simrace mod for multiplayer in beam. He implemented rudimentary tyre physics, tyre wear and stuff like that.. even different types of tyres, such as slick, semi slick, inters, wet etc.. hoping it goes well tbh..