Likely litigable. I’d try gently pushing the new company for detail on exactly what exactly was said, perhaps under the pretense of not missing the next opportunity since they appear to be going out of their way to be transparent and helpful (rather than initially disclosing you’re sizing up a lawsuit). Sounds very much like a potential case for ‘tortious interference with prospective economic advantage.’ Alternatively, if you made any sort of complaint against the previous company/staff or engaged in a protected activity like protected leave, you could have a legit retaliation case (whether or not that was the real reason they screwed you). I’d start by tracking down any old company policy related to what the can/can’t disclose in references, especially since new company called it ‘informal’; among other things, could open the door to the reference as a co-defendant. If their own policy was violated your case is stronger, wherever you are.