There's no difference between 'a law that says "this is illegal" but the authorities have no interest in actually pursuing' and 'a law that says "this is illegal" but the authorities have no interest in actually pursuing'.

My definition of "illegal" is "you can be arrested, charged, and convicted."

Intent and actors can change. A DA who says "I won't prosecute {x}" is under no obligation to continue, or even be consistent with, that policy, and the next DA is certainly not bound by it.