Is RequireComponent[typeof(CustomScript)] possible?

Absolutely. The script must inherit from MonoBehaviour and must compile though.

@Berenger In Unity 2020.3 it's normally works:

     [RequireComponent(typeof(CtrlVerticalSpeedMoveCharater))]
     public class MainCtrlStateMachine : MonoBehaviour
     {....}

Note. public class CtrlVerticalSpeedMoveCharater : MonoBehaviour and therefore it can be attached to GameObject.

If you talk about the "CustomSciprt" which isn't derived from MonoBehaviour (ScirptableObject or standard C# sciprt) in this case it doesn't have any sense, but "it "can be did by common c# technics.