in current versions of phpBB 3.1 the SphinxAPI is used to talk to a Sphinx Search Daemon. This API is deprecated as stated in the sphinxapi.php of current Sphinx versions:
// As of 2015, we strongly recommend to use either SphinxQL or REST APIs
// rather than the native SphinxAPI.
// While both the native SphinxAPI protocol and the existing APIs will
// continue to exist, and perhaps should not even break (too much), exposing
// all the new features via multiple different native API implementations
// is too much of a support complication for us.
// That said, you're welcome to overtake the maintenance of any given
// official API, and remove this warning
As a bonus the usage of SphinxQL could give a high performance gain as shown it this (even some years old) benchmark: