The main part that is being dropped completely is CUPS Browsing. This is currently the primary mechanism for CUPS-to-CUPS printer queue discovery on Linux. It works by having each CUPS server periodically broadcast UDP packets on port 631 announcing its available queues, and listening for broadcasts from other CUPS servers. This discovery method is being dropped because DNS-SD is preferred upstream.
CUPS on Linux can use Avahi instead, but this change means that automatic CUPS queue discovery with CUPS 1.6 will require Avahi running on both the server and the client.
In addition, CUPS 1.6 will drop several file type filters. These will be moved to a new package, so it should not be a big deal."