What I'm saying is that the purpose of newer versions is to provide bugfixes that exist in older versions.
I did however misunderstand the original post. This is your testing version, I thought you meant it was the version that was being run as the server.
Now, the entire swath of 1.8.9 versions can't possibly be completely unloadable, so a downgrade might work just as well.
But, no, you can't make a Sponge plugin that doesn't use the Sponge API. If the server owner already has a sponge server which works just fine, get the version he's using and use that to test.