Glad to see I'm not the only one concerned...
To my experience, it is not linked with Yosemite or iOS8 upgrades (at least not on the user side), as I have the issue between Macs running either Mountain Lion or Mavericks (or even SnowLeopard on an old iMac) and iPhones or iPad still running iOS7
My feeling is that the issue is on Apple's side and it may be linked to new features implemented on iCloud servers with the release, by exemple, of "Family sharing" between Yosemite and iOS 8...
To add another clue that it is not on user side that the source of the issue lies, I have noticed that simultaneously the old iMac running SnowLeopard lost the ability to display the events of the shared calendars (when it has shared these calendars just well for years!)
This iMac can still create shared calendars, invite people, create new events on the shared calendar but it won't display them on iCal. By example when a new event is created on this iMac, the event is displayed and as soon as the synchronisation is done with iCloud servers, the event appears on www.icloud.com, on Macs or iPhones sharing the calendar, but it disappears from iCal on the iMac.
I don't expect any solution from Apple for this iMac running SnowLeopard (as iCloud has never been officially implemented in SnowLeopard),
but for this lack of notification, at least for some users, I really hope it will be solved quickly!
