Which preposition is correct here?
- The truth about "Unlimited".
- The truth on "Unlimited".
The meaning of the sentence is roughly, exposing the truth of a concept, for example "Unlimited".
To me your use of "Unlimited" is ambiguous- Is it a company named "Unlimited"? Or do you mean the truth about the word 'unlimited'? (Note capitalization differences). Or do you mean the truth about the concept of limitlessness.
It think in general the truth about is used when discussing truths about a tangible entities. So if it were the company "Unlimited" I'd use the the truth about.
I typically use the truth on when discussing the less tangible: The truth on whether John's actions today helped divert the disaster may never be known.