I don't know the answer to your 6yo question, but my guess is that it's best to use the latest firmware. On my v1 Netgear R6400 router the latest is 1.0.1.78 (a/k/a "V1.0.1.78_1.0.50"), which was released in 2022. On a R6400 v2 router the latest firmware is 1.0.5.128.
I actually just dropped in here to praise the Netgear R6400. This seems to be the only thread about about it (except custom firmware discussion). That might be because it's so reliable that there's not much to say about it. It's boring. It just works.
There are two versions of the R6400, and mine's the older v1. I got it used, and I've been running it for about 6.5 years. In all that time I've only needed to reboot it twice. In both cases when it malfunctioned it was still routing correctly to the wired devices that I had plugged in, but in one case I couldn't access the UI, and in the other case, if I recall correctly, I couldn't connect a new device to it.
That makes this Netgear R6400 enormously more reliable than the TP-Link router which it replaced, or any other router that I've owned in the past.
I'm also impressed that Netgear did a firmware update for it seven years after its release.
Two years ago I switched to AT&T fiber Internet, and the new gateway supposedly would do everything I needed, so I tried to retire the R6400. I forward ports ports to different machines on my LAN, but when I configured the port forwarding on
AT&T's Humax BGW320-500 gateway, it never worked right. The solution (after many wasted hours) was to un-retire my old R6400, and put the gateway into "passthrough" mode, to the R6400.
One caveat: I've never used the USB ports on my R6400, at all. On previous routers which I've owned (including that TP-Link) I've found the USB functionality to generally be unreliable. So I didn't even try it on this one.
My R6400 is old, but it is still working well, and I'm in no hurry to replace it. When I eventually do replace it, my next router will almost certainly be a Netgear.