Heh, what a weird decision. The RT-AX56U and RT-AX58U were released at almost the same time. The AX-56U had better specs to my eyes, so I ended up going for that one a few years back. I liked that the radios were separate, so likely had a lower chance of overheating. Turns out if I had made a different decision, I'd still have a supported router.
See here:
https://forum.lowyat.net/topic/4658096/+4120
Or here:
https://en.wikipedia.org/wiki/List_of_Asus_routers#ASUS_Wireless_ax_Routers
Heh, well... I went high end originally and got the RT-AC3200, and it got discontinued pretty quick. Then I got the RT-AX56U, and it got discontinued pretty quick. It doesn't seem like there's much of a pattern. Some of the old *66U and *68U routers are still supported, making me think that there's no real pattern to it - it's just luck, probably based on behind the scenes complexity. If hardware is finicky, the engineers drop it. Otherwise they support it. Maybe a model that sells best gets supported a bit longer, because there's millions in the wild. It is weird though given all the similarities between models (in the RT-AX5x lineup) that the 56U was the one to get cut. Oh well. Looks like I'll be doing another router upgrade sooner than expected... once scripts/addons stop supporting it.