Hi JDulles,
L&L has one Windows developer, I believe. There may also be an additional part-time developer involved as well. The point being, this is a tiny outfit. It makes sense they won’t guarantee the software will run on the older OSs, because they don’t have the bandwidth to do so.
But it might work fine. You can always install new updates to try them, and revert back to prior if you experience problems. Scrivener updates are here.
Or, if the current version works for you, then stick with it.
Best,
Jim