Thank you very much for all replies!
Unfortunately, I was unable to bring back that checkbox via troubleshooting. I had to clean install win10 and that brought the checkbox back.
Since the checkbox went missing I've noticed a big drop in FSX performance. I thought that the missing DircectX 10 Preview was the reason because I always had it checked. However after installing windows 10 from the scratch and getting DirectX10 preview back the performance is still pretty bad. I'm not sure what happened but some time ago there was Windows update that could have caused performance degradation. I'm not a computer wizard so at this point I'm going to put the FSX aside.
Once again, thanks a lot for all the input!