The Tetris Company doesn't have any rights to the game, as Tetris isn't patented. They tend to claim they have "trade dress" rights, but the elements they claim as such in the Tetris Holding, LLC v. Xio Interactive[1] case aren't actually protectable. They're all either technical elements of the game (like the movement of the pieces, or the shape of the board), or visual elements of the game that do not have any recognizability (as in, if you removed them, people will still recognize it as Tetris, like the colors of the pieces). In the end, the only thing that makes people recognize Tetris as such, is the shape of the pieces, potentially the fact they fall down. Those are all technical elements, so they can't fall under trade dress. Trade dress is meant to protect things like the design of Coca-Cola bottles (curvy bottle, red label, white text), where said design language is very meaningful (you recognize the Coca-Cola section from far away just by the design), non-technical (any other bottle would work equally well, the product stays the exact same), and confusing if copied (if you picked one up but it turned out to be Pepsi, you would be well confused). Trade dress is not meant to act as a stand-in for a patent. Sadly, DMCA is very anti-developer in this regard. A small developer who relies on a publisher (like Steam, or the App Store) to publish and monetize their game, can have their game taken down in minutes with a (fraudulent!) DMCA claim, with no real way to appeal. If you do try to appeal, like in said Xio case, you won't actually have any money to deal with the following court case (which TTC will happily stretch as far as possible to bleed you dry), since while you are under DMCA claim, you have no audience, and no way to monetize. Due to that, you will have to pretty much withdraw from the case, which is what happened in this case. Had there been enough pushback from Xio Interactive in this case (which, would cost them a LOT of money), they could have won. It's a very skewed system.

To prevent this, TETR.IO is self-published (not on Steam, etc.) and has many pillars it stands on. This makes it nigh-impossible to take down with a simple DMCA request. To take down TETR.IO, you would need to start a court case. And then, the developer can keep their game up in the meantime, as then, the game is innocent until proven guilty, instead. The Tetris Company has not sent any legal message to TETR.IO, and I personally doubt they ever will. It's important to remember that if TTC loses such a case, they would go out of business. The only business TTC has is selling their (snakeoil) Tetris License. Such a license would become nearly worthless if it was proven the trade dress isn't real. So, they would probably rather just keep an eye on me. In fact, I wouldn't be surprised if they'd rather sell a license to me, as that would be far, far less risky for them, unlike sending an email to Steam.

[1] https://en.wikipedia.org/wiki/Tetris_Holding,_LLC_v._Xio_Int....