Fundamentals have changed, not been removed. Most fundamentals from the past do not work in today’s game.
QBs - drops changed - they are quicker/crisper, releases are quicker, throwing motions more refined. Kids are throwing better in younger ages, processing speed and football iq is greater
RB - more emphasis on complete backs, have to be able to run a route tree, higher emphasis on patience, setting up blocks, and being able to make proper cut. (Alex Gibbs run game didn’t get truly popularized til mid 90s)
WR - massive evolution from back in the day. They were in 3 point stances. The ability to create different releases, the stems in the routes, working off leverage points in coverages to get open. As a whole, massive evolution in the sport in WR techniques
OL - No flipper only blocking and heavy handed downhill blocking. Zone mechanics, gap schemes have changed on insert points technically, head placements on traps. Pass blocking mechanics are completely different. Have to be able to kick slide, vert set, jump set. This is a massive difference from the old days.
Defense - tackling has changed. You won’t see a high level coach teach head across. Too risky for concussions or shoulder injuries. You will see higher teachings of rugby tackles, gator tackles, and heart 2 heart profile tackles. To comment on the deal above when it comes to “ducking head”, never taught at high levels but in open field you are taught to run through the thigh boards of open field runners. Also to say that tackling is much safer today, and in the past they tackled with their head waaaaaay more than players today do.
DL - much less penetration, you play with your hands. Extension…length. You rarely see true vertical steps on 1st or 2nd down. NFL is slightly different because they are freaks. College is more controlled in that regard. Pete Jenkins is an 80s guy, and his theories are heavy in the game today.
LB - only a few teach down hill immediately, and that’s even evolved recently. Being able to freeze step, pop tops, and be able to cover is the difference. Have to be able to cover the pass.
CB - techniques never really changed, you can only pedal. T step or bicycle break so many ways. It’s harder today to play the position because of the rule changes. Have to be better technically to cover these receivers today. Can’t bully at the line and just grab and hold.
The game isn’t less technically sound. It’s just very different than the past. Comes down to blocking and tackling, but even those techniques have evolved and will continue to evolve.