It is not available via the AppCenter, and installing it manually results in an app that asks for authentication and then exits.
I wanted to use Synaptic to install Steam, as I had under Elementary OS 5.
Elementary OS is moving towards flatpak packages. It is highly recommended you install the flatpak version of steam.
Just open that link and press Install. Then it will open a Sideload screen that will let you install Steam on your system.