Back in my LE days, during the last gasp of service revolvers (and revolvers remaining popular for off-duty), most common OWB options for leather exposed the trigger guards, like the scabbards and paddles. The IWB designs, like the Pistol Pocket, naturally covered most of the trigger guard due to the nature of putting the revolver down behind the belt.
The clam shell required it be uncovered (due to release button location). The advent of the breakfront holster often included either a fully or partially covered trigger guard (nature of the push forward draw). My issued Hoyt was of the "partially covered" type, as the revolver was pushed forward, covering the trigger guard while transition from the holster.
Today you can find traditional designs, which don't cover the revolver trigger guard, as well as newer designs (like the Galco belt slot/scabbards) which cover the trigger guard.
The days of DA/SA revolver and 1911 carriers not worrying about open trigger guard designs were intended for older generations of users. The days of more modern service pistols, especially those with trigger safeties being an integral (and easily deactivated) part of the trigger, have understandably given rise to concerns about preventing ND's due to anything touching the trigger of a holstered gun.
When I found myself having to move through heavy brush while wearing an exposed belt holstered duty weapon, whether it was a service revolver or a DA/SA pistol, I was more concerned about something disengaging the thumb snap and levering against the grip, lifting the weapon from the holster.
Nowadays, with the advent of plastic pistols with "integral trigger safeties", I'd also be concerned about anything being able to reach into the trigger guard and come in contact with the trigger, so I want them covered.