I've been fly fishing for 60 years and have seen many changes. I don't remember 60's so well, but mostly since I wasn't a good fishermen. But, raw sewage, industrial pollution, and poor farming practices were obvious. After every rain my local streams smelled like a combo of manure and nitrate fertilizer. Dairy farms had their cows in the streams all summer to cool down. First Earth Day in 70 started the environmental movement, which along with decline in dairy farming IMHO led to stream improvements until about 2000 over here in Jersey where suburbanization pressures started reversing the gains. The same thing happened in the Lehigh Valley but 2 decades later.
More licenses back in the day which was mainly felt on opening day as everybody fished (no video game distractions back then), but still few good fishermen. Other than stocking days, pressure dropped off in late May. Most old pros were deadly with bait (salted minnow pros got a lot of trophy browns and guys who dead drifted salmon eggs with fly tackle loaded up on rainbows) and fly fishermen were rare. Fly fishermen were very secretive and it took decades to learn many skills. Fly shops were very rare so not much choice in tackle, flies or materials. Bucktail, Muskrat and pheasant tails were popular since everyone knew hunters and trappers (who knows trappers today?)
Fast forward, the bait pros are largely gone replaced by fly fishermen who can learn the sport pretty well learning from the internet and a few guide trips in a few years. Not every kid fishes any more, so opening day isn't the huge deal it once was. Licenses sales are down, but my guess of 20% of anglers who are good fly fishermen are out there a lot and have generally decent skills so the pressure many places see (especially the special regs areas) is greater and trout see a whole lot more flies than they used to. Going up to the unstocked brookie streams and catching a mess of "natives" for breakfast was always popular, but the support for wild fish in the bigger streams is something more recent.
The internet has also raised expectations. Back in the day getting your limit was a real treat after which you went home. 2 lbs trout were like unicorns. Now, it seems as if you don't get 100 trout a day or haul in a few 20 inchers in a morning it is a rotten day, based on what is posted on the internet. In the 60's many people were happy to get a brace (2) in a day for supper. Now, one has to get into double digits to not be embarrassed. I think some of this has led to the high/low hole mentality since some guys have to wring every possible trout out of a stretch of water.