As we know from the definition of fluid, it continues to deform under shear stress, but some fluids such as rheopectic, Bingham plastic, and thixotropic do not deform initially under the action of shear stress as in case of solids.
Should it be categorized into the fluid?
As correctly stated that Fluid is defined as a substance which continuously deforms under applied shear force. Even though only after exceeding a certain shear stress value rheopectic, thixotropic, and Bingham plastic start undergoing shear deformation, it is to note that after that stress value, these substances also deform continuously until the external shear force is removed. Hence, they can be categorized as fluids.
However, in case of solids under any amount of shear force, they undergo a fixed deformation and do not undergo continuous deformation.