iOS8 is the first version of iOS that allow 3rd party developer to deploy custom keyboard (aka Input Method). At first, it really concern me a lot about security. Because user can use that keyboard not only for texting or email but also Log-in and their credential will be exploit by the 3rd party keyboard.
But after reading technical document from Apple website. I feel a lot better.
Ont only that App can reject the use of any custom Keyboard except the built-in system keyboard provide by iOS. For example if you develop and Banking app that conform to the HIPAA privacy rule in the US. developer can do that by return value NO from method application:shouldAllowExtensionPointIdentifier: of UIApplicationDelegate protocol.
Custom keyboard cannot access to the text value in the copy "Clipboard". So don't worry, you can keep doing that safely..... for now. Although, I am not highly recommended to do so.