9 might have been the last one I have used. I definitely have used 8. (And certainly 1.2, 1.3 etc, but those were different times...)

Is there a concept that the search machine would find, which has brought the radical change you mention?

No, just lots of different things.

G1 has had ~700 improvements since JDK 8 and is now very different: https://archive.fosdem.org/2020/schedule/event/g1/

ZGC, the low-latency collector (which gives a ~1-2ms max pause times on heaps of up to 16TB in JDK 16) was introduced in 11 and made production-ready in 15.

In general, the way the GCs work is just very different from 8.

Plus, there have been lots of improvements to CDS and startup time in general due to how the VM loads classes and initialises data (~30 ms to run Hello, World on a cold VM in 15) -- https://cl4es.github.io/2019/11/20/OpenJDK-Startup-Update.ht..., https://twitter.com/cl4es/status/1311335253139771393, https://www.morling.dev/blog/building-class-data-sharing-arc...

And, of course, now everything lies on top of the module system.

Complete replacement of GC algorithms for start, with G1, ZGC and Shenodah.

Regarding OpenJDK, other JVM vendors have improvements of their own.