While shoes have played a role in faster times, I think there are two other factors that have had a big impact on the growth in the number of fast times
1. Kids are running and more importantly training much at a younger age than before. I ran my first 10k in 1987 at age 9 at the tail end of the running boom and it was rare to see more than a couple of kids younger than junior high age at races. Today I see kids as young as six running 5K’s and there are dozens or 9-11 year olds running races with clubs. By the time kids get to junior high many have been formally training for several years already.
2. Training quality , I remember reading about Alan Webb’s training in sports illustrated in 2000 or 2001 and was pretty floored because it was so much more intense than anything we did in HS just a few years earlier. Coaches took notice and that was just the beginnIng. The knowledge and understanding of training and nutrition among top HS coaches is vastly superior today than it was 25 years ago. Separately, and this is becoming true in most sports, top HS athletes now train nearly year round for their primary sport. There are so many more summer training camps, whereas it used to be common for students to play other sports in addition to running it is kore likely thst they top ones now go all in on track / CC.
In short, today HS runners are training much more like college athletes at top programs than they did in the 80s, 90s and early 2000s.