> Is there a risk those closed source projects would be sued following the release because they disclosed some forgotten sin?
I know nothing about law, but I guess software companies would protect themselves in advance by mandating that all their developers use only software and sources that belongs to them anyway. That would be useful in case of a lawsuit where a court could force them to show the sources to prove they don't come say from the claimant's reverse engineered software, or some other FOSS project. Things might be different with 3rd party modules and libraries, where an obscure module was licensed from a company that went out of business ages ago, but all their assets were acquired by another party who finds the module in the wild and decides to sue.
Just speculating, but although I'm 100% in favor of FOSS, if the above were real possibilities, I couldn't but understand why some companies are so reluctant to release even old code they couldn't profit from anymore.