If the author is not willing to unmask him or herself, I'm not convinced courts would allow an author to maintain anonymity while suing you for copyright violation. Particularly given the specifics of this case: the copyrighted work being freely available, and the author publicly stating that it's abandoned.
You can certainly keep on using it, and the license probably allows you to modify it on our own for internal existing projects. So it will be around for a little while.
But why try to keep the dead horse alive? TC is really weirdly written, as a GUI app that has a command-line stapled onto it, instead of the other way around. (Last time I built it for a headless machine, I still needed wxWidgets.) Especially with the specter, even unlikely, that some unknown person may possibly rise up to claim ownership of the project someday.
We should understand TC because we're using it, but we should think about the next thing.
If you have the right to copyright under a pen name, should you not also have the right to sue without revealing your identity? A right that can't be exercised doesn't exist.
Rights can come in conflict with one another, and when this happens one has to yield. The public has the right to open court records, and defendants have the right to know who's suing them. These rights usually outweigh any right the plaintiff may have in suing anonymously. It is possible to sue anonymously in the U.S. (e.g. "Jane Roe" of Roe v. Wade was a pseudonym) but IANAL so I don't know by what standard this is determined or whether the TrueCrypt dev(s) would qualify.
It doesn't matter. A licensing pall cast over the project severely hamstrings the future maintainers of the project; big companies, for instance, will be reluctant to get involved while there's a cloud over it.
