kioclient5 exec http://localhost:9000 now works correctly.
Not sure what you mean by "also works". Current code checks if the URL ends with :x:y and turns it into ?line=x&column=y. This is used to open local text files at the given cursor position. However, for URL's like http://localhost:3000 it is invalid to perform such transformation. My patch checks URL scheme and applies the transformation only if it is file://.
I suppose the kate developers like the fact that this currently works over FTP, SFTP, FISH, SMB, etc.
So maybe only HTTP[S]/WEBDAV should be blacklisted (because there queries have a different meaning, one that we can't know client-side).