I used to have one and it was declared. Depends on how your insurance company views it.
Some insurance companies view all exhaust mods the same, so you could have exhaust manifold, decat, cat back and its the same in their eyes as just haveing a crappy backbox.
Other insurance companies do not allow modifications which make the car illegal to run on the road, and would automatically include decat in this as most cars would then fail emisions tests.