dddd

This may or may not relate directly to the original topic, but at some point I believe (in the late '90's?) the loading/strength standards for production frame/fork testing increased sharply, meaning that steel framesets in particular became yet harder to produce to a competitively low weight.
Of course steel frame production as applied to better-quality bikes had already trailed off by then.

I recall in the early 2000's someone bringing me a new Tange Prestige frameset to build up, and my being shocked by how heavy it was, despite it not having lugs.

Another related data point of sorts would be Peugeot's adoption of lugless frame construction around roughly 1980, where similar bikes could be compared having lugged versus lugless construction. The lugless construction removed significant weight, allowing lesser frame tubing to be used without any net weight gain.
So as far as the "when" goes, that continuum would seem to have began in the 80's as far as the higher-production models goes.

Lastly, Schwinn was perhaps the biggest player in terms of the other end of the time-line, when large-scale production first started shifting to Taiwan.

The arrival of uni-crown forks would perhaps be another significant point on the timeline, many early examples of which were brazed using a bikini style of crown.