My Mini 4 is still on 10.x and is still buttery smooth. Some might ask why 10.x... first, it is the last version of iOS that is optimized for iPads. Subsequent versions of iOS have been iPhone-centric in the use of the screen. There are a few points of difference, but perhaps the easiest to recognize is the widget screen in landscape orientation.
Here's what the widget screen looks like on my Mini 4 running iOS 10.x
Here's the widget screen on the 12.9 iPad Pro running iOS 12.x
Another reason is support for 32-bit apps. I have apps that are 32-bit and no longer in the App Store (nor will ever return)... things like MAME.
Am I concerned about any security issues running iOS 10.x? No, and I won't be unless someone shows me that this Mini 4 is vulnerable in my day-to-day use.