While this script probably still works, I will not continue to support it. If you really must, use the -no-browser +open steam://open/minigameslist trick instead.
This script will download and install the last stable version of Steam before the Library update on 10-30-19 from the Steam CDN, and write a file to prevent the bootstrapper from attempting an update.
Before you start, close Steam. You can also (optionally) clear out all of the files in your Steam directory except for steamapps and userdata.
This is intended as a temporary fix1. Newer games, especially those that use the new Origin integration (i.e. Star Wars Jedi: The Fallen Order) may not work! More importantly, running old versions of Steam may be fine for a while... until an exploit is discovered. Be careful.
This script runs on Windows 7 x64, Windows 8.1 x64, and Windows 10 x64. The downloaded items will cause undefined behavior on other operating systems.
Open PowerShell (windows key + X -> PowerShell), and then
Set-ExecutionPolicy Bypass -Scope Process -Force; iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/antigravities/unreact/master/unreact.ps1'))
Download unreact.ps1. Then, type Set-ExecutionPolicy Bypass -Scope Process -Force
into PowerShell, and press ENTER. After that, drag unreact.ps1 into PowerShell, and press ENTER.