That post is 6 years old, and yet Linux still sucks by default on a touchscreen.

I installed NixOS on a handheld nearly 2 years ago and tried a whole bunch of touch-oriented options (including tiny ones like Maui). The closest I got to good enough was running the mobile fork of GNOME, using someone on GitHub's custom flake to pull it in at HEAD. I'm very happy that Valve has released SteamOS for other devices so I can offload that tinkering.

Steam is very usable on a touchscreen, but the KDE desktop mode hasn't impressed me.

I think the Steam Deck’s touch screen is also one of its worst features.

I will type a message or an email on a phone (Android or iOS) but I loathe even typing my username into the Steam Deck via touch.

I'm writing this from Librem 5 phone running PureOS with Phosh DE. It's convenient and pretty in my opinion. Also it runs desktop Firefox with all plugins you want.

You must use the plasma-mobile stuff, not the plasma desktop.

Mobile wasn't packaged for NixOS.

I am curious why/if Valve hasn't used it for SteamOS's desktop mode though.

Package it yourself then!

I don't really know, but I don't think the use outside of steam is an important use case to them, possibly they don't want to make things too easy, or maybe it wasn't packaged for arch either when they created the base image.