Modern versions of mcMMO expect you to be running the latest available Java Minecraft release, however older versions of mcMMO are still available and should work perfectly fine for those playing on legacy versions.
mcMMO is a plugin for the Spigot API, which makes running Spigot or Paper a requirement, other forks of Spigot/Paper may work but for those wanting a painless installation and having everything just work we officially recommend sticking to Spigot or Paper.
Historically very old versions of mcMMO ran directly against CraftBukkit (the spiritual predecessor to Spigot) but this is no longer the case and mcMMO will not work if you are just running CraftBukkit.
Upgrading mcMMO is meant to be as simple as possible, you are never required to delete data or configs.
Congratulations, you have upgraded mcMMO. Make sure you are running a compatible version of Java Minecraft and Spigot/Paper. The newest versions of mcMMO target the latest versions of Java Minecraft and therefor may not work with older versions.
If you have been running mcMMO for a while, chances are you may be on an older version of the config. mcMMO will for the most part respect user config settings, so it does not modify when they change unless it is necessary to keep mcMMO working.
Occasionally mcMMO will update its default config file values, and in certain config files if you do not add the new values yourself they simply won't exist. This is true for the treasure config files, the repair config files, and salvage config files.
If you're the type of server owner who does no customization of the aforementioned config files, you can simply delete the .yml files you find inside the mcMMO directory to regenerate the file with the latest default values. If you are the type who does customize their mcMMO installation extensively, then you will want to follow the changelog with every release and update values in these config files yourself as needed. Take care to not delete the mcMMO directory itself as that is where player data is saved by default.
If you are upgrading from mcMMO classic you will be able to follow the upgrade instructions and everything should work normally. However you will not be able to go back to mcMMO classic after you do this as the config files get updated to be compatible with the newest changes in mcMMO. A simple fix to this is to simply delete the .yml files when switching back to classic, and classic will generate new ones.
When using mcMMO against older versions of the game you must make sure to use a compatible version of mcMMO for that version of the game, and you must match this with the latest version of Spigot or Paper for that version of Java Minecraft. Legacy versions are as-is and any bugs/exploits found in them are simply left the way they are. No effort is underway by mcMMO devs to maintain legacy versions at this time.
If you encounter any errors/issues running Legacy versions, make sure that you have the latest Spigot (from BuildTools) or the latest Paper. If issues persist, you are likely out of luck as legacy versions are as-is and receive no updates, however mcMMO source code is freely available on GitHub so those with a programming background can fork older versions of the codebase and fix any problems themselves.