Open 3D Engine (O3DE) from the Open 3D Foundation is what was once Amazon Lumberyard, now open source it's just had a first major stable release. This 21.11 version comes as a result of work done together with the likes of Adobe, AWS, Huawei, Intel, and Niantic and hundreds of community contributors.
“With the first major release of the engine, developers and content creators can get started faster—and have confidence that the core components of the engine are stable and supported,” said Royal O’Brien, GM of Digital Media and Games for the Linux Foundation. “Engine builders, 3D simulation developers, and metaverse developers can use O3DE’s open standards and wide range of content creation tools to build the multiplatform 3D experiences of the future, today.”
On the subject of Linux support, it's now officially in the preview stage for both the editor and runtime.
Lots of features were added to the engine including performance profiling and benchmarking tools, an experimental terrain system, a Script Canvas integration for the multiplayer networking system, and an SDK to facilitate engine customization with platform support for Windows, Linux, MacOS, iOS, and Android and much more.
See the official announcement for more.