Which Sphinx do you want installed? There are various apps that are all called “Sphinx”. There’s a search engine, a doc generator, voice recognition, home automation, etc.
Sometimes old plugins that have not been updated and do not report compatibility with the latest WP version will still work, or work with some minor corrections. Of course such use becomes your own responsibility and is unsupported by the original authors.
You could manually install Sphinx, then develop your own integration with WP. The source code of old plugins can still provide general guidance on what you could do.
If you are after the search engine Sphinx, their wiki lists a few WP plugins that are outside of the official WP repository. One of those might be more up to date than the official listings here.
