How long would you like to be able to access this content?
If it is for the long haul, plain text is the only way to go (org-mode, etc).
Any proprietary commercial solution will fade away in a handful of years. At best, you might be able to extract the content before they shut down but structure and metadata may be lost.
Open source self-hosted solutions will last much longer but even in that case it might get to a point the source has been abandoned so long that it becomes difficult to keep going.
Only plain text will survive. I have plenty of notes in a more or less organized directory tree, dating back to the late 80s.
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 Why would that be outdated?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.
If it is for the long haul, plain text is the only way to go (org-mode, etc).
Any proprietary commercial solution will fade away in a handful of years. At best, you might be able to extract the content before they shut down but structure and metadata may be lost.
Open source self-hosted solutions will last much longer but even in that case it might get to a point the source has been abandoned so long that it becomes difficult to keep going.
Only plain text will survive. I have plenty of notes in a more or less organized directory tree, dating back to the late 80s.