Note that some subjects (e.g., law, history) are not so prescriptive in terms of required courses in high school (see Admission requirements for 2021 entry | University of Oxford as an example). STEM subjects are usually more prescriptive.
But the general point of narrowing is spot on. My D22 went from taking 12 GCSE subjects in Years 10-11 (freshman-sophomore) to 4 A-Levels in Years 12-13 (junior-senior).