Does LFS simulate aerodynamics while you are sliding (and could you make a car with ultra big fins in the new mods support to increase that effect)? Does downforce increase if you lower your car (or rather does upforce decrease)? Im really intrested in how detailed the forces are.

Press F when car is on track,you'll see forces view - if you take default LFS GTR car (XRR for example),you'll see at front,middle and rear of the car bars rising with increasing speed - these are forces from front wing,undertray and rear wing accordingly. Test around in different conditions,check forces in replay and you will have plenty of answers already.

I just tested it a few days ago by going in the air sideways using a ramp... it didnt seem like it simulated those sideways forces... i wonder how much it would impact rally time attacks if it was added. Maybe the forces are so insignificant IRL that they didnt bother with it.

LFS aero currently works purely in the longitudinal direction. Drag and lift/downforce is generated in relation to the car's forward speed (forward being the exact direction the nose is pointing - some setups have raised the front for lower overall drag). There are no additional lift and drag coefficients for going sideways.
Raising or lowering either end of the car has no huge aerodynamic effect. You'd basically need real-time fluid simulation to have something like that simulated.