I've looked into the iCloud settings on both my Mac and iOS, but I can't seem to find an option to disable this feature. Is there a plist entry somewhere that I can use, or is it forced on?
- Mac: Go to System Preferences > General > uncheck Allow Handoff
- iPhone: Go to Settings > General > Handoff > uncheck Handoff
Just so you know, Handoff uses a secure communication channel to transfer information between devices signed into the same Apple ID.
Data does not travel to Apple's servers or even over the internet. Devices advertise availability to each other via Bluetooth and data is transmitted over Wi-Fi, but only on the local network. However, once the data is on the clipboard on a computer, another app could then "sniff/copy" the data and send it on / store it. Basically macOS clipboard isn't secure from other apps installed, but handoff itself is very secure in transit.
There is a way to disable Universal Clipboard without disabling Handoff, if that's what you really want:
Open your Terminal
ClipboardSharingEnabledis enabled (equals 1)
defaults read ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled
defaults write ~/Library/Preferences/com.apple.coreservices.useractivityd.plist ClipboardSharingEnabled 0
In order for this to work, your Terminal must have access to Files and Folders or Full Disk Access.
Go to System Preferences -> Security & Privacy -> Privacy (tab) -> Full Disk Access (sidebar)
Click the padlock icon at the bottom left corner of the preferences pane and enter your account info (you must have administrator rights)
If the Terminal is not in the list on the right, click on the (+) button at the bottom and select it at /Applications/Utilities/Terminal.app
Make sure Terminal is checked before leaving