The way I estimate the situation (and I admit this is not a rigorous scientific conjecture), is the following (for public schools in the US in the average):
Early years: bad pedagogy, bad retention rates (ie, quitting after 5th grade to go work on the farm, bad average results, basically only the top 10% learned deeply and went on to intellectual pursuits
1900s - 1980s: Decent and improving pedagogy (the aforementioned procedure-first style for math), good and increasing retention rates, good parental and societal pressures to perform, great average results, top 50% or more went on to intellectual pursuits.
1980s - 2010s: Same math pedagogy, but with rapidly deteriorating parental pressure to perform, leading to worse results. A truly terrible detour for reading instruction (from phonics to context-based reading) that decimated the average reading level of those currently under 40. Currently being fixed but not yet replaced in all schools. See: "the science of reading".
2020 - 2024: an earnest effort, gaining traction and fast-tracked after the educational disaster that was COVID, to find curricula that actually work with current students.
The concept-first, teach-them-how to think approach for math really is pretty new, and only just now being rolled out in a lot of states.
In reality, a vanishingly-small subset of American students has ever been given an entire education using evidence-based instruction and curricula. Looking at what actually works and trying to synthesize it and scale it up state- or nationwide is truly a brand new experiment, and one the decentralized US education system is sort of designed to prevent. So we'll see.