Currently I have my macros synced through Dropbox, with all Macs using KM 9.2. If I upgrade one of the Macs to KM 10, can I still sync? Or do I need to upgrade every machine to KM 10? I have an older Mac that is still on 10.11 and I have to leave it there, so that particular Mac cannot get to 10.13 to run KM 10.
Obviously the macros that use KM 10 features wouldn't work in 9, but for the macros that DO work in 9, could a mix of 9 and 10 work? I guess an easier way to say it is, did the file format change between KM 9 and KM 10.
My experience of this is I get a warning if my Mac's Keyboard Maestro app is at a lower version number to my other syncing machines. The warning dialog says something like "Live Dangerously?". So, it is possible but not advised for reasons of incompatible sync data.
So if you want to sync macros, you need all your versions of Keyboard Maestro to be identical, and if some Macs are running pre-10.13, then that means either - you abandon syncing on those older Macs or you stick with Keyboard Maestro 9.2.
The “Live Dangerously” means your macros will be out of sync, and the only time you should do that is if you are just about to update Keyboard Maestro, or you are willing to allow your macros to be out of sync (and possibly lose some changes) in the period until they are all back to the latest version you are using.