How secure is my iPad Air 2 running 15.8?

I have an iPad Air 2 running the most recent version of iOS supported, which I think is 15.8. I have a newer iPad which is for daily use, but I’m wondering if it’s safe from a security standpoint to continue using the old iPad strictly as a local storage device. It’s literally never connected to the internet (WiFi and Bluetooth are both always turned off) and is only used to access files stored locally on that iPad. Most of the time, it’s turned off entirely. I kind of hate to ditch it entirely since it’s in great condition but don’t want to keep it around if it’s just a magnet for hacking due to an old version of iOS. Thanks.

[Re-Titled by Moderator]