Also macOS[1]. It was part of their upgrade to use a newer DirectX version[2]. Rather than maintain two renderers, they chose to end support for OSes which don't have DirectX support. If it costs more to maintain support for a platform than that support brings in, why would they do it?
Yeah, not only that but Linux users are known to be hostile to commercial software companies.
Given that, the microscopic market, and the additional programming complexity I don't see a reason why any game company should go out of their way to support Linux ever.
(As a Linux desktop user, I'm thankful when they do though. I'm so glad that Beyond Compare from Scooter Software works on my main workstation, for instance. Also Zoom, Slack and VS Code among others... Looking forward to trying MS Edge some day soon.)
[1] https://support.rocketleague.com/hc/en-us/articles/360042201...
[2] https://support.rocketleague.com/hc/en-us/articles/360043307...