What they go after is not just the name, but the shapes. But I think you are fine as long as you do not monetize or get popular.

The name is the most important part, as it gives them a way to look for games that have their name but aren't theirs. But you're mostly right that they actually go after any game that has the same gameplay and is relatively popular, but only when they can easily issue a DMCA takedown. It's not like they ever said that this was their policy but you can see it bear out time and again.

TETR.IO and Jstris are insanely popular games that have been up for years with no action against them since they aren't hosted on another platform/marketplace.

This is also why Apotris got 2 DMCA takedowns on both Github and itch.io 3 months apart from each other, but after hosting the game on my own site, they haven't taken any further action in the past 2 years while it has continued to grow in popularity.

The shapes are called tetrominos, four squares connected orthogonally. Tetris the game, was created with pentominoes (5 squares) and scaled down to tetrominos. The game was inspired by physical puzzles where fill a rectangle with the shapes. So these shapes existed before Tetris.

Would they go after the shapes? Possibly. See "Tetris Holding, LLC v. Xio Interactive, Inc.".

One I remember is Blockles by OMGPop, which was a great multiplayer tetris-inspired game. There was legal threat, and they took it down in 2009.

Which is crazy, because the shapes are just an idea, and you can't copyright ideas. They also couldn't be a trademark infringement (IANAL, though) if you don't use them in your branding.

> you can't copyright ideas

They can go after you for anything. Whether or not that would hold in court is a separate question, but they count on the fact that you would be probably unwilling to test it.

Yes, legal trolling is unfortunately a thing.

It is the series of shapes in one project that makes it Tetris. So think of it as a series of copied ideas in one product that make it a legal challenge

Trying to copyright shapes or blocks of various shapes is wild to me.

They don't claim copyright on the mechanics. They pursue trademark violation. If it has falling tetrominos, it might be confused with Tetris, is the idea. You can use different shapes and be fine.

I could imagine the specific combination where the shapes colors match the original could get into trademark territory, as the colors have no in-game requirement to be those specific values.