Writing Software to Last 50 Years
Text files are king! I store every single byte I can in text files. Examples:  - Tabular data     : TSV   (almost all Un*x/GNU tools handle this out of the box)
  - Simple "records" : GNU Recutils format (https://www.gnu.org/software/recutils/)
  - Formatted texts  : Markdown, LaTeX, ...
  - ...

If I need some hierarchical kind of information, I use a folder structure to handle this.I know that not everything can be stored as text. But I try to use open, well documented and future proof formats. Examples:  - Images : PNG
  - Music  : FLAC, Ogg, ...
  - If I really need to preserve orignal format/design of a web page: PDF

Nothing's perfect but stay away from any closed/obscure/proprietary formats.