Copy and paste in clear text? Yes, I don't think that's a good idea. Download to disk in clear text? Yes, I don't think that's a good idea.
Years and years of security incidents with consumer data show that this is a really bad idea.
At minimum, a credential manager distributed for wide use should encrypt exported/copied keys with a user selected secret or user generated key.
> At minimum, a credential manager distributed for wide use should encrypt exported/copied keys with a user selected secret or user generated key.
It feels like this stated minimum is not your actual minimum.
Consider for example a macOS user keychain. The keychain is encrypted on disk with a user-selected password. But once you unlock the keychain with the password, you can copy and paste passwords in clear text. The keychain is not a black hole where nothing ever escapes. And I have no objection to this setup; in fact it's my current setup.
So when you say copy and paste of passkeys in clear text is not a good idea, there's nothing inherent to encrypting credentials with a user key that prevents such copy and paste. There would have to be some additional restriction.
> At minimum, a credential manager distributed for wide use should encrypt exported/copied keys with a user selected secret or user generated key.
What should happen if the developers refuse to enforce this?