Late 1960s through the 1970s.
Hundreds of titles that survived into production even though "prohibition" existed.
That is an accomplishment in itself.
This includes the earliest of solid state machines.
The 80s, 90s, 00s, and today are not even close to the variety and features that were developed.
Two words, "backbox animation", a lost feature in most games, but occasionally still brought back under the simplest of methods.
Yes, that it correct, short, long, and mini flippers, bumpers, ramps, spinners, spinning disks, captive balls, locks, rollovers, magnets, vari target, roto-target, carousel-target, and shaker motors all existed long before modern games.
I can give multiple examples of every feature I just listed.
Some features are now non-existent, or rarely appear like the vari target on RBION.
Innovation, "zipper flippers", active targets, spinner targets, and a host of other items that were listed above.
Every time you look at a wedgehead or final runs of EMs you are looking at the grandfathers of modern game innovations.
If Steve Kordek was still alive, he could explain so much that has already been lost to time.
Concepts were all reused to the modern games of today, but generally LESS than they were in the 1960s/70s.
For example, BoP spinning face is a modern modification of the older roto-target concept, it is not a new gadget.
The primary advancement of more recognizable games people know is in sound, music, ability to development deeper ruleset to tell "stories", and visual scoring with backbox animations. More powerful flipper coils and electrical current conversion allowed more vertical reach on playfields as players wanted "more", and as Roger Sharpe would say players "wanted it all".