Had a look today...that plug is for the ace system, it goes to a junction box under the passenger side door.

 

Most manufacturers' wiring harnesses support all available options. It's less expensive to design, manufacture and install the same harness in all versions of a model than to do so separately for all variations, trim levels, options, etc. If you keep looking you'll likely find many bridge-to-nowhere connectors.

It's quite rare for an ACE truck to be converted to non-ACE. A lot of serious suspension work is required, I believe.