I'm still able to use my USB keyboard to write letters, I didn't use it anything else I think. I used it to write some letters to villagers just today.
I plugged my keyboard into one of the USB ports in the front of the dock and can use it to type. You might also be able to do more with it, but I haven't tested it any further. I previously also used it for the ingame chat but switched to the chat via a phone and NookLink.
it's odd, because I am currently using a wireless keyboard which was working fine but just stopped working on me. I think it has something more to do then just AC though. Tested it on my PC and worked fine, but it just won't work on my switch, even tried rebooting the console and changing the USB port with no luck.