Developer Psyonix made the announcement earlier today on Steam.
Rocket League

Psyonix, the developer behind the once very popular Rocket League announced that they will no longer support macOS or Linux come early March. It is in early March that a patch will be released for all three platforms (Windows, macOS, and Linux). This will be the last one for every PC platform but Windows.

As stated, the announcement came first via Steam. In the announcement, the team says that while they are ditching support for those on Mac or Linux, they will curiously still support those running on Windows 7 or newer.

Windows 7 hit end of life on January 14, 2020.

They also say that this abandonment of macOS and Linux is because they are continuing to upgrade the game "with new technologies." However, they have not said what these new technologies are. The only changes made to the game over the past several updates have been the addition of more cosmetic items. In fact, a recent point of controversy with the Rocket League community is how the price of unlocking newer cosmetics had gone up significantly.

All of this comes about half a year after it was announced that Epic Games had purchased Psyonix. The game was initially slated to be pulled from sale on Steam in "late 2019," but that window has obviously come and gone. You can still, at present, purchase Rocket League through Steam. It's not yet clear how much longer this will be possible though. Shortly after the acquisition announcement was made, Epic released a new statement saying that they had no announced plans to stop selling Rocket League on Steam.

If Rocket League is indeed going to be pulled from Steam still, I'm personally anticipating that this change will happen after this last multi-platform patch is released in March. I figure that after that update is out, they can safely pull the game from sale on Steam. It would then be exclusively for sale through the Epic Games Launcher. This way, Psyonix won't have to deal with supporting macOS and Linux on a platform they are no longer selling their game on. It also makes sense when you realize that the Epic Games Launcher only supports Windows fully, macOS partially, and has no native Linux support.

Curiously, Unreal Engine 3, the game engine that Rocket League utilizes, already supports macOS. If it was a matter of the engine not supporting what they want to do, their excuse for ditching macOS in addition to Linux doesn't hold much water with what we know.

I guess we'll just have to wait and see what happens. The only thing we know for sure right now is that if you purchased Rocket League on either macOS or Linux, you're SOL. Get those multiplayer games in now while you still can.

We want Rocket League to be the best experience possible for all our players. This includes adapting to use new technologies. This has made it more difficult to support macOS and Linux (SteamOS). Because of this, we will have a final patch for these versions in early March.

The macOS and Linux (SteamOS) versions will no longer be updated or supported after the final patch. You will still be able to download and install these versions, but some features will not function as expected.

If you purchased Rocket League for Mac or Linux on Steam you can download the Windows version. This version will run with full functionality on a PC (personal computer) with Windows 7 or newer.

Players on Mac can try running Rocket League on Windows with Apple's Boot Camp tool. Boot Camp is not something Psyonix officially supports. You can learn more here:

If you play Rocket League on Linux, you can try Steam's Proton app or Wine. These tools are not officially supported by Psyonix. Please see the links to both below.