You should always be blocking Nintendo Updates via DNS if you are running CFW. Yeah the WiiU is old, and nintendo "probably" won't release another firmware update for it, but that is just a best "guess". The truth is, no one knows if they will release an update besides Nintendo themselves. So its better to be safe then sorry. And if your running CBHC/Haxchi/Mocha or any CFW, and your console updates to a new unhackable firmware, then the update "could" brick your console.
At least I am pretty sure it could brick your console anyway, but I could be wrong. That may be if you have CBHC installed. I don't know. I rather be safe then sorry, and not chance a brick.