I have never heard of anyone shortening shock absorbers and I am not sure how you would go about it. The shocks are threaded at the top where they go through the mounting (or the bottom if you have bilstein struts). To shorten them you would have to make the thread longer and I doubt if the rods are mild steel. So it would not be easy
The shock absorbers do not affectt he height of the car, it is only the springs which do that. When you hit a bump hard enough the car will run out of suspension travel, but this is softened by the bump stops, which stop damage to the car. The shock absorbers paly no part in this.
When you lower the car it just sits a bit lower on the shocks, the piston is a bit lower in the bore than at normal ride height. And tha's it. Your car will run out of suspension travel sooner than before, but I have never known that be a problem on road use.
After all if you had to change the length of the shock absorber every time you changed the ride height how would you manage with adjustable platform sprongs like you find on alot of competition cars. They will change the ride height in the middle of an event in a few minutes if need be.
