It is my experience that pack weights were fairly light in the 60's and 70's for experienced backpackers. The 80's had a lot of people with little experience buying big internal frame packs (heavier than externals) and loading up with gear at REI. You cannot compare those "rookies" with seasoned UL hikers on BPL. Compare the BPL hikers with the experienced backpackers of the time.
Here is what I used most of the time in the 80's for 3 seasons. Gear was similar in the 70's for me too.
Kelty D4 Pack 3.5 lbs
Poncho Tarp 1.0 lbs
Blue Foam Pad 0.75 lbs
Down bag 2.5lbs
Plastic ground sheet .25 lb
Total "Big 4" = 8.0 lbs
Gaz Globe Trotter Stove/pots 1.25 lb
First Aid Kit .25 lb
Military Wool Watch Cap .15lb
Military Wool Gloves .15lb
Sierra Designs Wind Pants .70 lbs
Sierra Designs Wind Jacket .63 lbs
REI Down Jacket 1.25 lbs
Spare Wool Socks .25 lbs
Spare Liner Socks .07 lbs
Lexan Spoon .03 lbs
Military Compass .35 lbs
Pocket Knife .09 lbs
Space Blanket .25 lbs
3 Nylon Stuff Sacks .25 lbs
2 Nalgene Bottles .50 lbs
Iodine Tablets Needles, Thread, Cord, etc. .50 lb
Maps .25 lbs
Empty GT106 Gaz Butane Canister .13 lbs
Coffee Container .06 lbs
Plastic Trowel .13 lbs
Matches/BIC Lighter .01 lbs
Total Gear in Pack = 7.25 lbs
Base Weight = 15.25 lbs
I think most people consider 10-20 lbs lightweight. On many trips I was even lighter, sometimes around 10 lbs.
In the Complete Walker III Colin Fletcher had a two-day one night gear list that included:
FSO = 24 lbs 9 oz
BaseWeight = 16 lbs 11 oz
Notice that my Kely D4 weighs 3lbs 8 oz and I did two 6 month trips with it in the early 70's. Compare it to two of the popular PCT packs, a ULA Catalyst (3 lbs 11 oz) or a ULA Circuit (3 lbs 0 oz).
Dehydrated foods/instant foods/Pemmican bars have been around for over 50 years. Instant Oatmeal even longer... our food was not much heavier than today. If you read Colin Fletcher's "Thousand Mile Summer" you will see that he averaged 1.4 lbs of food per day, and he resupplied about once a week. This was in 1958 and he hiked for 6 months, mostly in the eastern deserts of California.
Yeah, we carried 40lb packs, but that included food for 10 or more days and a couple hundred miles. None of this re-supplying every 3 or 4 days.
Fletcher's total pack weight for a week in the deserts was 44 lbs, excluding water. He carried 2.5 lbs in canteens and 4 lbs in camera gear for the newspaper articles he was writing. His largest item was a 5lb 10 oz down bag. Again, remember that this was in 1958 and backpacking gear was extremely hard to find. Subtract his food and cameras and his base was 30 lbs. Again, this was in 1958!
I find it amusing that we think we have re-invented lightweight backpacking. Now, SUL was probably nearly impossible in those days, but there were people hiking with frameless Rivedell packs in the 70's... so maybe some did it. In the 70's the typical shelter I saw on the trail was nylon or plastic tarps... influenced by Fletcher who rarely used a tent. Used to see an occasional plastic tube tent. Our shelters were probably lighter than the average tent used today in the UL world. Down was a problem, because of 500-650 wt was the norm and heavier nylon shells were used. GoreTex was around in the late 70's, but you usually saw ponchos on the trails back then. GoreTex was for mountaineering. Leather boots were the norm, but not rare to see people hiking in Converse, Purcell, or Keds tennis shoes.
Here's another one, since I talk to Dan McHale occasionally. In 1969, when he was a teenager, he did the JMT unsupported in 11 days. Used a Camp Trails external pack. Total pack weight was 40lbs including consumables.