Unfortunately, there isn’t much you can do. This is mainly because Apple don’t provide a way to install older versions of an iOS app. So you can’t choose the older version.
The options are the you stop using the older device with Agenda, and just use the newer iOS version, or you turn off sync on the older device. The latter is not very useful, of course. It would only work as a sort of archive. You could export notes from that device, so maybe you could see it as a scratch pad.
Note that none of this is really specific to Agenda. Nearly all apps will at some point drop the oldest OSes. Some do it much more aggressively than we do. When this happens, anyone with a device that can upgrade the OS can do so. Those with really old devices may not be able to. This is a shame, but also the nature of our industry, which puts a usefulness date on most things.
Yes. We actually wanted to make this very explicit, so we started communicating it in the app itself in Agenda 14. People who were on an old OS which would be affected were warned with a dialog, which is the best way to get to people we know about. (Eg. a web site posting is unlikely to be seen by anyone.) Of course, it is no guarantee. You could miss the dialog because you aren’t using Agenda for a while on that device, or see it and just forget about it. It’s the best system we could think of, but not infallible.
Note that although we have updates that do require an update everywhere, because we have added a new feature that changes the data model, this is the first time we have changed the minimum OS requirements in the 5 years since Agenda came out. So it is quite rare. Standard updates can require that you update Agenda, but will not require an OS or device update. We try to announce this in the release notes, but a lot of people don’t read that of course.
We certainly have tools for this type of rollback, but they are not generally available. It would be something we would have to do for you if you were willing to send your data.