Linear compression springs are the most commonly used in the spring industry. They are a lot easier to design than a conical or barrel spring would be because a linear spring’s rate is constant. A tapered spring’s force, on the other hand, is not based on a constant increase per inch of travel due to the different outer diameters per coil which compose the tapered, cone, or barrel spring.
There are other customizable adjustments you can make to a linear spring besides changing the diameters of its coils. You may also adjust the pitch (rise angle of coils). Spring pitch is highly involved in your spring’s force and travel capacities so having a variation of pitch between your springs coils will definitely alter your springs force, turning this into a non-linear spring.