I asked the same question a few months ago.
If you can switch over to the ath79 target, it is probably an opportune time to do it since the next OpenWRT release won't support ar71xx. Keep in mind that you need to manually recreate some of the configurations -- use a backup for reference, but don't restore files blindly (on my device, the /etc/config/network file was the only one that I needed to manually restore, but review/compare the files to avoid introducing problems into your fresh 19.07.0 install).