Oh, time flies!
It is (almost) 20 years since introduction of first “real” Maven Proxy: Proximity, that later became Sonatype Nexus (1 and 2, not much to do with 3, it happened after I left Sonatype):
https://lists.apache.org/thread/vp9ghlncvvnozcs3mqh9ghsmmrksyoh5
Proximity was the first MRM that implemented “group repositories” (in other lingo “virtual repositories”), that since then, proved a huge mistake. So let me apologize here for “inventing” groups: Sorry, and I mean it!