Omni Group has announced the termination of Calendar integration with OmniFocus, see:
http://www.omnigroup.com/support/omnifocus-ical-sync
Over at FileMaker, they are also announcing the forthcoming end of such integration with their product, Bento.
I've recently been talking with the folks at Fantastical, whose software integrates with Calendar. Here's their reply concerning the sync issue.
"Apple has deprecated Sync Services, but their more modern equivalents (CalendarStore and EventKit) continue to work. We don't expect to have any trouble interacting with Calendar in the future. I hope that helps, let me know if you have any other questions."
Makes me wonder what the real answer is.