Are we allowed to use or put shadows in designs that we usually submit here?
It is allowed but design must be realistic and satisfy some parameters, it all depends on what design you are create…
E.g. if you create mockups shadow is required to make item look realistic but if you create business card design for print then avoid shadow and gradient and focus on modern design.
This is a very vague question. It depends on many factors. While minimalist design is trending right now, shadows can sometimes enhance graphics.