I am running 19.07.4 on my Asus RT-AC58U since a few months back. I was going to upgrade to 19.07.5 when this popped up. The sysupgrade file that I found came from the firmware selector: https://firmware-selector.openwrt.org/?version=19.07.5&id=asus_rt-ac58u
What should I do? To be honest, I'm fine with just running 19.07.4 for now, but I'll probably want to upgrade at some point in the future and I'd like to know what this all means.