1970s UK: we had the Schools Maths Project (SMP) GCE A level syllabus and textbooks available for teachers to choose which used sets, mappings, very simple group theory and the notion of an inverse and an identity for operations and which included computational approaches (pseudocode!) and a lot of emphasis on reasoning and proving. There were plenty of more 'traditional' syllabuses (syllabi?) available as well.

My secondary school went for SMP maths (I suspect because the deputy principal was part of the project). I'm very glad they did. It helped later.

A longitudinal survey of the destinations of those who did A level Maths in the 1970s tracking who did the SMP syllabus and who did a traditional syllabus might be interesting...