What exactly was done in 15.x?
If the client is being written using the Open Client SDK, you can implement
error handlers that don't output errors to the user.
The options for other clients (third-pary tools, etc.) depend on what options the client application developers have defined for you.
From the ASE side, with the exception of a few specific messages, there is no way
to suppress sending messages to the client.
-bret