It depends on the 3d printer. Object Connex 3d printers, which use an inkjet to spray resins of different stiffness can do this. A number of different stiffnesses are possible by mixing different hard and soft resins together. Really the only limitations to how sudden the changes in stiffness are with this approach is that the manufacturer restricts you to only nine different material types. Oh and the manufacturer won't work with you ever again if you hack the printer drivers. This approach has been used to make soft robots with graded stiffness too[0].

[0]https://gmwgroup.harvard.edu/pubs/pdf/1255.pdf