We like to support things as long as we can. However, eventually we do see the need to drop older os support so we can focus efforts on current versions. The long life of Apple devices means we end up supporting very old hardware for a long time. For example, we still support the iPad mini 2 from 2013 running ios12. Thankfully apple also provides OS updates for a long time and everyone upgrades. There are actually a very small number of devices from 7-8 years ago that run iOS 12 but not 13 or 14. We are dropping support for these devices after the next release because ios12 is different enough that it needs a different push server and code and has increasingly required special debugging to deal with differences in the way the os works. It appears there are now only 24 ios12 devices actively using Monal. Apple stats are opt in only so there are definitely more but this number will still be much much smaller than the thousands more using iOS 13 and 14.
That means Monal 5.0 will be the last version to support iOS 12!
But we try to pack as much features into this release as possible to provide a decent xmpp client even for iOS 12 users.