30 years ago would be 1976. troff was around then and is still in limited use.
TeX is the poster child of a format meant for longevity. It is about 25 years old, and has undergone no change other than minor bug fixes in about 17 years. While it is not a mainstream format, it has some very well established niches and shows no signs of relinquishing them. (One thing that it is valued for is the fact that its specification remains the same down to the visible wavelength of light.)
If I wanted to write an electronic document that would have a long life, personally I'd use TeX. The storage medium that it is on might age, but the format will continue to be supported for a long time to come.
Cheers,
Ben