I've seen the claim that 'Tetris Effect requires Steam despite being an Epic Store exclusive' going around this week. It sounds like a hilarious case of platform dependencies conflicting with Epic's exclusivity strategy, but it's hyperbole: partially true, with a lot of emphasis on 'partially.' You don't need Steam to launch Tetris Effect, or to play it with an Oculus Rift.
The situation is this: Tetris Effect can be launched from the Epic Store without any extra software. If you want to play it in VR, however, you need a VR interface running, which can either be SteamVR or Oculus VR. If you were using an HTC Vive or Valve Index, you'd run SteamVR, which is downloaded from Steam but doesn't actually need Steam running to launch.
DSOGaming says that SteamVR is required even if you're using an Oculus Rift, and that Epic has confirmed this. Having tested it myself, SteamVR does not launch when I choose Tetris Effect's Oculus VR mode. I asked Epic myself, and was told that "you do not need [SteamVR] for Oculus." I haven't received any feedback on why there are mixed messages.
The situation is slightly awkward for Epic, sure, though that's what I assume Valve wants when it comes to SteamVR. It created VR tracking software that any developer, software or hardware, can interface with. It named it after Steam and made it available on Steam. Except for a small number of users who are going to tinker with other open source VR software, most will use either SteamVR or Oculus's software to configure their VR headsets. That makes Steam a natural place to buy and sell VR games, because anyone using a SteamVR-compatible headset has it.
For the foreseeable future, any VR game that isn't an Oculus exclusive is probably going to include SteamVR as a launch option regardless of what store it's available on. SteamVR auto-launching with a GOG game was an issue for at least one user.
That means that if VR really takes off, Valve has put itself in a great position by tying VR functionality, not just VR games, to Steam. SteamVR also supports the Oculus Rift, making it even more essential.
I found a much funnier consequence of this while researching the situation. I wondered if Vive-compatible VR games on Uplay also launched SteamVR, so I bought a copy of Eagle Flight. It appeared in my Uplay library, and when I went to download it, the Steam install prompt popped up. I had actually bought a Steam version through Uplay. Now that would be a funny way for Epic to sell Vive-compatible games, but it's not quite what's happening here.