I believe this is because of your iPhone IOS version because the apple AppStore won't allow you to download the latest version of the Mycelium since your ios version is not compatible with their latest one. It only allows you to install the last Compatible version with your current IOS version.
My suspicion as well. But because of rumors that Apple cripples old-generation phones in new IOS versions, I would abstain from updating 3 major versions up form 12 to 15, but simply buy a new phone instead (or just use an Android phone, which doesn't have any of this BS).
Partially you are right. The problem with development for android phones is huge fragmentation, there are dozens of OS versions used by people all around the world and at some point developer must decide for which versions he wants to target his soft - as a consequence some group of phones with older OS will not have access to the program.
I guess something similar we have now. OS 12 is already quite old and developer decided to target only phones with newer OS. Nothing to blame Apple for. And in fact I would not blame Mycelium too much too, as I know how annoying/difficult/time consuming could be maintaining old versions "just to have them". Adding critical features only in the new versions? so-so.
On the other hand - the latest iOS 15 is used on more than 70% of phones, the latest android on 25%.
If phone allows - update it, it could be only better.
https://www.xda-developers.com/android-distribution-numbers-2021/https://www.lambdatest.com/blog/ios-version-fragmentation/