Between two or more known apps, a solution already exists - don't put in the system-wide shared buffer, put it in a buffer whose access is restricted to apps that you know. For example, on iOS you've been able to create team-local (only accessible from apps with the same team ID) and app-local (created with a unique identifier so only your app knows & can interact with it) clipboards since...a very long time, definitely pre-iOS 8 because that's when I encountered the API the first time.

To some extent, a solution does exist for arbitrary (iOS and Android) apps, which lets you explicitly pick what application you want to receive some data - the sharesheet. It's criminally underused in my opinion.