Incredibly well written blog covering everything! Was worth a read for me. I was searching about expiration time of Apple's Universal Clipboard and stumbled into this.
Not having a universal representation of clipboard content is really unfortunate. I was working on something related to copying text and too thought "How Apple Universal Clipboard ensures that sensitive data is not copied over devices" followed by, "If it doesn't copy sensitive data won't the experience be hit or miss: sometimes working, sometimes doesn't?" Probably a convention to hint not copying like "CanUploadToCloudClipboard", having one to hint that paste is not allowed "NotPasteable" and the OS showing a notification on the machine when a person tries to paste is would've helpful.
Another problem is we do not have a stable and complete API for browsers (see https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/write). So if you're using a password manager extension and you copy a password from there, you're probably left with a plain text with no hint whatsoever.