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.
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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.