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.
Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ
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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.
My god that might just work, I've used RS Toolkit a bunch and forgot it had that feature!
Without mod tools or running an alternatively acquired copy, likely not
Yeah I figured this was the case. Ah well!
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.