Only the Steam snap may establish this interface. See Introducing early access to the Steam snap for more details.
The source code for the Steam snap: https://github.com/canonical/steam-snap
The test code can be found in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/steam_support_test.go
The source code for the interface is in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/steam_support.go