I would think requiring opt-in for clipboard functionality would be the more radical option that would leave most users (myself included, I would imagine) scratching their heads when they can’t copy/paste as a matter of course. Maybe you meant something more specifically related to 3rd-party sniffing/modifying clipboard contents, but I haven’t really encountered that outside of apps such as CopyQ and Paste, and they are pretty explicit and intentional about their functions.
I have found a lot of utility with cross-device copy/paste. I know it requires the somewhat mysterious phantom Bluetooth/Wi-Fi connectivity that AirPlay/Airdrop use, so if I have disabled Bluetooth on my device, for example, it will no longer work. I could see where it might not be fully reliable enough to count on, I have experienced inexplicable failures, not often but enough to understand that it might not be some folks’ default preference. As part of the “handoff” function, it can be disabled in Settings at least.
I have found a lot of utility with cross-device copy/paste. I know it requires the somewhat mysterious phantom Bluetooth/Wi-Fi connectivity that AirPlay/Airdrop use, so if I have disabled Bluetooth on my device, for example, it will no longer work. I could see where it might not be fully reliable enough to count on, I have experienced inexplicable failures, not often but enough to understand that it might not be some folks’ default preference. As part of the “handoff” function, it can be disabled in Settings at least.