Of course,devices on the newest version of iOS will need the later versions of macOS to connect and sync.
Yes, and no. What they actually need is the newest version of iTunes. New versions of iTunes will usually run on older versions of MacOS (used to be OS X).
You may need a new version of Mac OS for other features like AirDrop and/or Continuity. But only if you are running something older than two or three years on a Mac of similar age. Older Macs may not have the hardware to support some services.
Any service that syncs through iCloud now should keep on working. New services like iCloud Message syncing and group FaceTime may or may not work with older versions of MacOS. Apple has been known to update older versions of MacOS for minor service upgrades, eventually. Usually when they do a security update.
There were no announcements of new cross platform services, so there is no need to install a newer MacOS to make your device work. Just install the latest verison of iTunes when it comes out; probably the same time as the new devices come out.