The TCP_BACKOFF_INIT and TCP_BACKOFF_MAX socket options were never part of mainline Linux. They come from the "Customizable TCP backoff patch" that Ben Woodard and Brian Behlendorf posted to netdev in September 2006 for LLNL's oversubscribed gigE clusters, where thousands of TCP streams backed off in lock step:

      https://lwn.net/Articles/202398/

This support is definitely broken and unused at this point.