Rule R4 in chapter 10.4 of the 2019 game manual states “ROBOTS may not extend more than 30 in. (~76 cm) beyond their FRAME PERIMETER.” does that mean that my robot is illegal if it has the mechanical capability to extend past 30 inches? what if we keep it from extending past 30 inches with programming?
You can constrain it with software.
Some of the blue box text was added in team update 3: 
I think a software stop which limits extension should be fine? But to be safe, I’d recommend you have a mechanical hardstop on your mechanism to physically stop it at 30 inches as well.
You can constrain it with software but you will probably get a note passed along to the field that you have the ability to go outside of the 30" and will be watched a little more carefully