It appears that Capcom did not abandon on-disc DLC for Resident Evil 6.  YouTube user, FluffyQuack, has released multiple videos featuring the on-disc DLC for the game that has been unlocked via modding.  Consider this your warning for the following videos as they do have spoilers for the game.

First is a comparison video between the Pro difficulty and the on-disc DLC difficulty, No Hope.  You can see that there is a huge difference on damage received from enemies. 

Next is a costume and melee showcase that features the various costumes and attacks that can be unlocked.  Included in this video are some interesting throwbacks from the early PSX games.

The final videos shows all the multiplayer taunts from playable characters that are on-disc.  What's interesting is that there's a soldier character that is not one of the main characters of the game.  This could be a costume for another character or simply an unlockable character.

Although this content may be available for free by Capcom, I'm sure many gamers will be upset by the news that the on-disc DLC practice is still happening. Also, there could be additional content that could still be locked away that modders are unable to access.

Update: It's been brought to my attention that most of the content is unlockable via multiplayer, RE.net, and completing the game.  Per the YouTube user, there are partial files of DLC that is on-disc but not complete:

Here's additional information about each multiplayer DLC mode by looking at files on the disc:

Onslaught: Sounds similar to Slayers from RE5 in that's it's basically a competitive mercenaries, but it's played on 2 seperate maps. You can cash in your combo to send more enemies to your opponent's stage. There's a medal for killing your opponent with monsters you sent by cashing in combo. And a medal for sending all possible monsters, so I guess there's a maximum. You can send normal enemies and hidden bosses to your opponent. I think this mode is 2 players maximum.

Survivors: Similar to last man standing, supports both free for all and team based. Your goal is to kill other players. When dying you spawn as a monster, you can kill 2 players to spawn as a human character again. There's mentions of using enemy points to spawn as a tougher enemy, I dunno if that's for this mode or something else. Amount of points earned depends on weapon used. Whenever you get hurt, other players get to know your position. When there's one human player left, he wins.

Predator mode: One player is Ustanak, other players have to kill him as normal characters. Melees work better than weapons to rescue players from Ustanak. You can respawn when playing as a human character, but it is game over if all players are dead at the same time (I think Ustanak also wins if time runs out). As Ustanak, you can can switch between different arms (claw arm, wrecking ball arm and shotgun arm).

Siege mode: Team based hostage mode. One team play as monsters, other team play as human characters protecting civilians. As a monster, you can accumulate enemy points in order to spawn as tougher monsters. Human players can give orders to civilians. Enemy creatures respawn very quickly. Monster players get to know position of civilians when they are hurt. There are bonus crystals you can destroy, I don't know what they are for.

Update #2:  CVG has recevied this statement from Capcom regarding on-disc DLC:
 

"We still have unannounced DLC for RE6 that will be revealed in the coming months and while virtually all such content will not be on the disc there will be one piece of content, that for technical reasons, requires the use of a combination of newly downloaded data and data that is included on the retail game disc."

 

Source:  CVG