A bit of web searching revealed https://www.reddit.com/r/kdeneon/comments/ie97i1/cant_install_steaminstaller_on_a_clean_install_of/g2frxw6/ where I found a solution:

$ dpkg --add-architecture i386
$ apt update

That worked.

I guess you need to enable multi-arch by default?

I definitely second the idea of enabling multi-arch by default. It really confused me that I was unable to install Steam on KDE Neon. After I was unable to install Steam via apt, I downloaded the deb from Steam's website, but that did not result in a working installation (even after adding the i386 architecture and letting Steam download a bunch of needed libraries). The installation process went fine but Steam would always crash when started.

Using the technique described by you still doesn't work for me though. I did
$ sudo dpkg --add-architecture i386
$ sudo apt update

but when using
$ sudo apt install steam-installer
I still got
Reading package lists... Done                                                                                 
Building dependency tree       
Reading state information... Done
Starting pkgProblemResolver with broken count: 1
Starting 2 pkgProblemResolver with broken count: 1
Investigating (0) steam-installer:amd64 < none -> 1.0.0.61-2ubuntu3 @un puN Ib >
Broken steam-installer:amd64 Depends on steam:amd64 < none @un H > (= 1:1.0.0.61-2ubuntu3)
  Considering steam:i386 -1 as a solution to steam-installer:amd64 9998
Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 steam-installer : Depends: steam (= 1:1.0.0.61-2ubuntu3)
E: Unable to correct problems, you have held broken packages.

I was able to simply install steam directly via
$ sudo apt install steam
After switching to a new console window and executing
$ steam
the other needed libraries were downloaded and Steam is now starting on my system.

Note that when I do
$ sudo apt search steam
I get
funnyboat/focal,focal 1.5-11 all
  side scrolling arcade shooter game on a steamboat

python3-iapws/focal,focal 1.4-2 all
  Python3 implementation of the international APWS-IF97 steam tables

steam/unknown,now 1:1.0.0.67 i386 [installed]
  Launcher for the Steam software distribution service

steam-devices/focal,focal 1:1.0.0.61-2ubuntu3 all
  Device support for Steam-related hardware

steam-installer/focal,focal 1.0.0.61-2ubuntu3 all
  Installer for Valve's Steam digital software delivery system

steam-launcher/unknown,unknown,now 1:1.0.0.67 all [installed,automatic]
  Launcher for the Steam software distribution service

steamcmd/focal 0~20180105-3 i386
  Command-line interface for Valve's Steam

Note how all packages include the "/focal,focal" part except for the steam package and the steam-launcher that have "/unknown,now" and "unknown/unknown" respectively.
I'm not sure but maybe this is why installing steam-installer still doesn't work?

Furthermore I want to mention that the Synaptic package manager does only list the packages
- steam-devices
- steam-installer
- steam-launcher
when searching for "steam" (so it doesn't list the steam package itself). Only now that I have installed it manually, does it also list "steam:i386".
Not sure of this info is even related to the problem, but in case it is I wanted to share it :)

We don't support ubuntu packages. I've confirmed that Steam's actual steam package they advertise on their website is working as expected. Whatever remains is an instance of ubuntu clutter -> https://phabricator.kde.org/T13500