It's not particularly surprising that all your devices need to be updated, how else would it work? The whole point of E2E is that the ends are your devices.
Right, but it may be unexpected that a single device can prevent using a new feature on your other devices. This is just a heads up. And conceivably Apple could provide updates for older OS versions, as they sometimes do for security fixes.
This has been the case for other iCloud features and they've historically done a good job communicating this to the user at the time they upgrade the service and when they attempt to access it from an old device. I would expect that to follow the same process here either refusing to enable it until your devices are updated or having the old device kicked out until it's updated.
