Long term storage of simulation data is usually happening in tape archives. These tend to have a significantly longer lifetime than hard-disks, however, are not meant to be constantly accessed. Simulations that are still in active use should be (also) kept on hard disk.
Usually, the data that should be archived needs to be gathered in a single (.tar) file. Consequently, a file containing the subdirectory structure should be created and kept for record:
tar -cvf mydirectory.tar mydirectory
ls -lRt mydirectory > mydirectory.toc
Then both, .tar and .toc files should be archived. It makes sense to keep the .toc file on the working computer to have information about what is stored on tape.
Smaller data files
For smaller files, a more modern cloud solutions which can be continuously updated might be a better way to go. However, to my knowledge this is not a good idea/expensive when exceeding several TB of data.