Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

You can download specific DLCs but I'm not entirely sure they'll be useable. It's worth a try. Here's how you do it:

Open a web browser and type in steam://open/console and it should open the Steam console inside of Steam itself. Type download_depot <game_id> <depot number> for Rocksmith the game id is 205190 and you can look up the specific DLC you want on steamdb

So for example, the Pearl Jam Song Pack could be downloaded using download_depot 205190 222139

Rocksmith's DLC SteamDB page: https://steamdb.info/app/205190/dlc/

If this DLC has been disabled as you said, this might not work, or you might need to make sure that Steam doesn't try to update the game (which would disable the DLC again). You can do that by browsing to ~/.steam/steam/steamapps/ (assuming you're on Linux, or just wherever your "steamapps" folder is on your PC) and find the file called appmanifest_205190.acf and set it to read only. That should prevent Steam from modifying any changes you've made to the game. Just remember to set it back to writable if you want to update the game again.

The game might not recognize the DLC as I said, but you should be able to access the files anyway if that's all you wanted. They'll be in ../steamapps/downloading at least until the download is complete, then it should try to apply the DLC to the game, which might cause the files to get deleted. I'm not entirely sure about that.

yes. I own the game and have used the repacked DLC fine. I was using CDLC before though so I'm not sure if you need to sort that out first sorry

Are you on windows or linux, if you managed to fond the dlc files, you can most likely(not 100% sure it works with delisted) use creamAPI to make steam think you on them. On windows I've used CreamInstaller, which is a handy GUI that does it for you. I seem to recall I did it on my father computer running ubuntu, but don't recall exactly how.

All the CDLC for RS2014 work by pretending to be the DLC for "Smashing Pumpkins - Cherub Rock", because it was given away for free as a preorder. If you get any of the other delisted DLC, you can use Rocksmith Custom Song Toolkit to change the ID to one you own (free or paid, as long as you have a steam license for it) and it should work.

Without mod tools or running an alternatively acquired copy, likely not

2014...

If you want something that far "out of print" those repacks are your best bet. The only other options that MIGHT be available are key reseller sites. If you're lucky they have some keys. If they still work is another problem.

Anecdotal but I moved somewhat recently and found my box of old consoles/games. I found my copy of Marvel Vs Capcom 3 (PS3) and booted it up to find out that I never claimed the DLC. I bought the steelbook Special Edition with a DLC code for the extra characters. Tried to redeem and apparently the code expired. If I want the characters I have to buy the "Ultimate" version of the game which has the characters on the disk.