I am wondering what information from 1988 might still be of help for you now, but I would suggest that xml and json or similar would still last about as well as plain text, but would enable you to do more interesting things and store more useful documents (say a complete contact list).
XML or JSON are also plain text. And Org has excellent support for storing contacts.
Furthermore, a personal knowledge repository may contain information that is pretty much timeless. Say you read Rudin to learn analysis in 1988 and took some notes. Why would that be outdated?
Some academics build huge perosnal knowledge repositories that they keep growing during their careers. These will often span longer periods than 1988-2019. See e.g. Luhmann's Zettelkasten.
The advantage of Org is that it is very readable, as it descends from an outliner, and has excellent tooling. There are options for including tables, equations, bibliographies, links to many datatypes, attachments, extensive date support, etc. Lots of export tools, interpreters to show views of your document, and many other things I have not touched upon.
Yet it is quite simple hierarchical plain text. So it will remain accessible in the future. Aside, Emacs has been around for very long, and will probably remain around much longer due to the Lindy Effect.
