The game's code can't be updated to the latest Unity version purely because of the save/load system. We are using a library which hasn't been updated for a few years, and so doesn't work with modern Unity. The other library option we tried, which has been updated, doesn't work for our project, and doesn't give sufficient error messages to allow us to understand why. We simply don't have the time to write our own fully recursive serialiser to replace the libraries, so were forced to use the old version.
If you were to remove the save/load feature, the game should be able to be updated to the latest Unity version. Certainly all the code would still be valid.
I assure you, I wish we could use a more up-to-date Unity version, but this was the only way we could get a save/load system working, and we considered that to be critical.