This. I've had the mini 4 since 2015. I've had it on 10.3.3, and while it can take iOS 15, I am not going to update it at all. In fact, if I could get up to 10.3.6, that would have been great, but that's okay. The reason I am not updating is because I still have some classic games that I play that have not been ported to being totally 64bit (iOS 11 is 64bit only).
I still am totally enjoying playing Midway Arcade, which has the classic Midway/Williams early 1980s arcade games, like Joust, Robotron 2084, Spy Hunter, Root Beer Tapper, Gauntlet, Gauntlet 2, 10-yard fight, Arch Rivals, Rampage, and more. I also have on it the classic version of Paperboy, Crazy Taxi, and the Street Fighter II Collection (Street Fighter II, Street Fighter II Champion Edition, Street Fighter II Hyper Fighting Edition). None of those have been ported to iOS 11 and have been abandoned. As such, my Mini 4 will never be updated past iOS 10.3.3.
When I get the mini 6, I'll be plenty fine with it being on anything new like iOS 15, and I'll move everything that doesn't require iOS 10 over to it, but for now, my Mini 4 is serving its purpose completely, and completely free of any cosmetic or spec change everyone may be up in arms about between the mini 5 and 6.
BL.