I'm not sure there was ever a day where the vast majority of hunters ever had a real chance at a 400+ inch bull in Utah. Yes, they are still killed every year. Yes, you might be able to get one. Yes, there are multiple units where these happen. But no, it was never a common occurrence, even in the "good old days." (Which really are right now for elk in Utah on many units)
Boulder has been putting out some giants the last few years, including multiple 400+ inch bulls. They seem to come on the archery hunt more than others, due to the guides that are killing them with clients early in the season. San Juan probably has the highest concentration of "big" bulls, but not guaranteed to produce a 400+ inch bull. But might be the best chance for a giant for the average hunter. You might have to "settle" for a 375 inch bull, though. Monroe has kicked out a 400+ bull each of the last few years, but the overall numbers are down from what I understand. Dutton still has a big boy or two running around, including the giant killed a couple years ago on the statewide tag. Beaver, Pahvant...those have potential too.