Turtles can feel throughout their shells however their shells are covered in a layer of keratin (similar to fingernails). However something to consider is that turtles can tell when something is touching their shell, and they generally do not like it. This can cause some turtles to act aggressively.
Turtles also do not have nerve endings in their shells but touching it causes a shift in the tissues that keep it attached to their bodies. This can be compared to the way you cannot feel your hairs or fingernails being touched but rather them being pushed or pulled from the tissues that they are rooted in.
Basically turtles are able to feel through their shells but they will not feel a lot of sensitivity during it. Sensation does transmit through the turtle’s shell however for many this is a sensation that they would prefer to avoid. Some turtles do however enjoy being scratched in the joints between their scutes, especially when they are growing.