MUMPS still sees a lot of active use today -- and MUMPS databases store data in plaintext, in ways that tend to be pretty well-documented and relatively easy to understand. EBCDIC, Betamax, laserdisc, Zip drives, and HD-DVD are probably better examples.
Didn't you say that MUMPS is an obsolete or generally weird format, not that it's something you wouldn't use?
But yes, I use it, both professionally and personally. Legacy MUMPS is not pleasant -- the VA's code looks like someone left their telegraph out in the rain -- but the language is astonishingly good at string-handling, mostly due to certain constructs that make it very hard to translate into normal languages.