Supported Since Release 19.07.0
Supported Current Release 19.07.7 -- this is the line I am confused about. Does this mean I cannot build more mature versions?
Okay, it seems I misunderstood the version scheme numbering, then? Is 19.07 a major release?
Edit: this https://openwrt.org/about/history page clarified things. I was under the impression that different routers had varying levels of version support. This clarified things for me.