icloud syncing between different versions of the same app

I have usually kept one Mac on an older system version while upgrading the others to the most recent system.

Apps built into the system should work with iCloud and sync well. I have never noticed problems with Calendar, Notes, Mail , Contacts, even with the Mac and iPhone and iPad on different systems. Each system will just show, what it can show.

The most differences I noticed with the iCloud photo library on different versions of Photos and macOS X. For example, when the migration from El Capitan to Sierra changed the camera filters to a completely different set of filters, the photos with the older filters applied have been replaced in the new version of Photos by new image files with filter burned in. We had to revert the photos to the original to be able to edit further. On the older system I could not see the filters I applied on the new Mac. Usually the older system cannot handle the features that are new in the newer versions, but iCloud Sync will do some conversion.

This will be different for applications, that are not part of the system or third-party application. usually newer applications can open documents created by older versions, but older versions of an app may not be able to open documents created by a newer version.

I have not seen any deletion, just not a perfect sync to the older devices. But to be on the safe side - make a current backup of all devices before upgrading. We need a full backup anyway before an upgrade, and with iCloud in the bargain it is even more important, in particular, if some data are only in iCloud because of "optimize Storage". then you need a separate backup all iCloud data as well.