I was not around for the glory days but I know Marty could tell us what it was like.
Back in the day it was not uncommon to catch 2 or 3 20+ Browns during a good hatch. My largest fish taken from the Provo was a 24 inch Brown. The fish on average have gotten smaller and the big fish are few and far between. They reconstructed the middle sections, taking away a lot of the deep pools. They also keep water flowing year round. They used to dewater a lot of the middle section. The Upper Provo has not changed and the Lower Provo still produces as it has for the 40 years I have been fishing it. What has changed the most is the sensitivity of the resident population of trout. Back in the day size 12 was a common size for a nymph. Now you have to fish 18 - 20s.
This bait section on the Middle Provo is my favorite. For some reason, most of the fly fishing crowd leave it to the bait slingers. The past few weeks I have had days with little to no pressure from other anglers. They do not plant the middle and lower sections of the Provo. There are however lake run rainbows and browns coming up from both Deer Creek and Jordenelle. The Provo is an awesome fishery.
As for swinging flies in the spring, there are a few factors to take into account. Fish on rivers like the Provo, because of fishing pressure will hide in the deeper pools. They do venture out when there is a strong hatch of midge or BWOs, but for the most part, if it is bright the fish will not be in the fishable swing water. On cloudy days it is game on. Two days ago I was fishing the Lower. Never saw a fish until the sun went down behind the mountain. The next hour or so every fish in the river was up taking midge and spinners off the surface. For the fun of it, I swung up a few. they were on the grab as well. It is all about comfort. Big hatch, cloudy day or after the sun goes down the swing is on, even during the winter.