There will be ar71xx devices under v19.07 as the last release for them, as I understand https://openwrt.org/meetings/hamburg2019/start
Early experiences in porting to ath79 are described on Porting guide ar71xx to ath79? The gist of it is that a proper DTS file needs to be created. There is no "cookbook" for this, though the device's "mach" file provides many of the clues as to what is needed. There are many ath79 DTS and DTSI files available for examination in target/linux/ath79/dts/ Once a "base" DTS file is available, changes such as "repartitioning" flash or RAM size are reasonably straightforward.