Actually it never got to the FBI (despite the media reports). There simply weren't any death threats made (again despite the numerous media reports). Doesn't mean many UK fans didn't cross the line. Hopefully they have learned from this experience.
As far as Mitch Barnhart, Higgins has only reffed two regular season UK games. First was UCLA last season in a CBS game which Barnhart may not have had much control over. Second was a SEC game vs. Vanderbilt last season, which because it's a league matchup Barnhart had little to no control over. (and FWIW I don't know for sure but I doubt Barnhart takes time to worry about specific referee assignments. More likely someone in the basketball office pays closer attention to those sorts of things.) The fact Higgins reffed a UK-Vandy game is strange too because I didn't know Higgins reffed SEC games. (He's primarily been a Big 12 official once conference season starts.)
Link to record of John Higgins' UK Games
The rest of the games were all in the NCAA tournament, which UK really has no control over.
One thing which needs to have more transparency and better decisions is who decides which refs advance to the Final Four and what is the criteria. Whoever decided that Higgins deserved to advance to the Final Four after the UNC-UK debacle should be fired. (and I doubt it was Barnhart).
I'm not expecting to see Higgins show up in any regular season UK games going forward. It could happen in the NCAA tournament, but that would be a mistake since Higgins filed his lawsuit (which includes claims against John Calipari), it would seem to be a conflict of interest for him to ever do another UK game again. Hopefully that stands forever, because he can't be trusted to call a fair game, even before the lawsuit.