libsodium doesn't put any metadata (including library and/or construction "version") in any its results, like boxes, keys, etc. So, there is no mechanism for versioning and backward compatibility.
So, it is so-so Ok for real time communication between two installations of software product (and even then two ends could have different versions of software and/or library used!), but it is not suitable for long-time storage or communication between different systems.
You store your "boxed" backup [keys] on disk. You want to read it 15 years later. How could you be sure, that libsodium didn't change algorithms in these 15 years? You need exactly same version of library that created this "box".
And even application which is aware of this problem could not properly solve this: it could record & check libsodium version and refuse to work with "old" data, but how could it enforce future libsodium to read product of old one, as there is no any provision for this in API?
libsodium is too simple for many tasks.