The contents of your clipboard _can_ be directly related to the functionality of an app.
For example, a link saving app like Pocket might check if your clipboard currently contains a URL when you open it. That allows the app turn a slightly tedious operation (tap/hold input field to bring up context menu, tap paste, tap button to save) into a single tap ("save copied URL?").
Whether or not the convenience is worth it might be debatable, but I fail to see how one would call that nefarious.
There a APIs in iOS (which existed before iOS 14) that allow you ask the OS if the clipboard content matches a pattern (e.g. is it a URL) that doesn’t trigger the warning in iOS 14.
It does appear that lots of apps don’t use these APIs, the developers probably never knew the existed till now, but there is a privacy preserving method of the building the functionality you talk of.
Aren't they just checking with an UTI? So for example a package tracking app can't ask if it contains strings matching patterns describing common package tracking code formats?
True (and I'm aware of those APIs). Just pointing out that "I want to know what's on the system clipboard without the user explicitly pasting" does not automatically equal trying to hoover up your data and phone home with it.
The maker of the Apollo Reddit app chimed in on the reddit thread that his app would check to see if a reddit URL was in the clipboard and offer to take you to that page.
Chrome uses it so the URL appears when you select the address bar.
Some apps detect if you have a 2FA auth code copied and auto-pastes it. I think Discord might be one? I can't remember which, but I've seen it on at least one app.
I can totally expect others to detect copied URLs that may belong to the app's domain and then offer to direct you to that particular URL (for example, I think the SomethingAwful app on iOS does that - if it detects a forums.somethingawful address it'll offer to load that particular thread for you).
That said, I definitely want to see more visibility about when and why this is done. Apple are absolutely in the right to show me a popup whenever it happens, so apps are forced to be clear and transparent about it.
For example, a link saving app like Pocket might check if your clipboard currently contains a URL when you open it. That allows the app turn a slightly tedious operation (tap/hold input field to bring up context menu, tap paste, tap button to save) into a single tap ("save copied URL?").
Whether or not the convenience is worth it might be debatable, but I fail to see how one would call that nefarious.