To what extent is the HOST system still in use?
From this news article dated April 30, 2015:


  "As of today, Host is history," FAA Administrator Michael Huerta declared during a press conference.


From the latest NextGen report dated June 2016:


  In March 2015, the 20 planned ERAM sites achieved operational readiness, which signified the full commissioning of ERAM into the NAS and allowed the FAA to begin decommissioning the legacy HOST system.


So, it is fully out, it is not available as back-up, and no ATC system uses Host.