Almost seems like 60's. Of course it could be my memory LOL
I thought 70's ushered in the new ground breaking VLF technology, and then the icing on the cake was TR Discrimination LOL. I still remember buying my first TR Discriminator / VLF detector. I believe it was either a Garrett or Tesoro. I was killin it! That thing went all of 4-5"
All-metal VLF was 1974. And not widespread since it afforded no discrimination. So ... unless you had a place that afforded the liberty to dig all (no way to pass nails), then it wasn't widespread.
Then yes: TR disc. got added to them in about 1975-ish. And not widespread till a few years later (1976 to 77-ish)
And then disc. VLF (aka GEB) got introduced in 1977 with the Red Baron, and the 6000D soon followed. Yet, again, not widespread till nearly 1980.
Keep in mind that in the era before the internet, word traveled much more slowly. So as good as some of those advances were , yet .... people were slower to discover and swap to them. When I started in 1975 or '76-ish, there were even still people swinging BFO's. And by 1980, as groundbreaking as VLF disc. was, there were some of us still swinging TR disc., who simply didn't know any better.
Thus all the "ground-breaking" advances you cite, are really like late 1970s, in popular usage spread.
Brings back memories of my first detector I got at an auction in the early 70s. A BFO no one wanted. I did not even know what it was . Just a detector that I can find things with. You talk about digging a lot of junk !! You got a lot of silver back than too. Times change.. KEN